You're not wrong in principle but this is a hugely broad and multifaceted issue. I'll try to cover a few different points in cliff note form.
*The group the WaPo's criticising (and rightly so) is an advocacy group. You get these on all sides of the aisle and to be fair they don't (usually) claim to be journalists. When the media reports stats from groups like that they need to be careful what the organisational bias is and be transparent with it. The other problem is that groups like that tend to be better resourced than a lot of people so that gives them a disproportionately loud voice in the public discourse. Those groups are different to journalist organisations though, and with the rise of internet platforms it's getting tougher to draw a distinction.
*The broader issue is that social media is a petrie dish for misinformation or details out of context. In that particular case it's easy for someone to see the "18 school shootings this year" headline stat and are more likely to share it (and more importantly trust it) if they already have the "guns are bad" idea in their head. Any jackass with an opinion now has an outlet that can easily reach millions with pretty much zero oversight for what's true and whether an appropriate context is provided. And interestingly enough the Russians essentially weaponised that in the 2016 election. A lot of the charges Robert Mueller laid out a few days ago were for people involved in troll farms who'd go out of their way to spread deliberate misinformation (ie, actual fake news and not some nebulous term used for someone presenting facts you disagree with) that would resonate among certain social media bubbles. The single most shared article during the election was the one about the Pope endorsing Trump (an outright lie) and an article correcting that rumour got half as many shares, which demonstrates how much faster a lie can grow in that environment when people WANT to believe it. I wish I had an answer to that, and to their credit Facebook and Twitter seem to be trying to find fixes but I'm not holding my breath any time soon.
*The readers already have the "guns are bad" idea in their mind and they're more receptive to that fact. Not only are they less likely to take a critical view of those facts, but they also go out of their way to look for facts they agree with. This is a human nature thing (essentially using brain power takes energy so learning to take shortcuts is an evolutionary trait that worked fairly well way back when) and is EXTREMELY well documented. It's called "confirmation bias" by the people
@Willbacker looks down his nose on. But people do need to learn to take every fact they come across with a pinch of salt, ESPECIALLY the ones that have a conclusion they agree with.
*Journalists in mainstream organisations are doing it a lot tougher than people think. It's not a secret the kind of structural headwinds print media in particular is facing, and speaking from experience a journalist's "agenda" usually doesn't involve much more than rushing around trying to get a story filed under deadline. There are certain things that they should do (and some people and some organisations are better at this than others) such as asking for a response where appropriate (though that's another complex issue in itself because anti-vaxxers for example don't deserve to have every media outlet asking for their opinion just because they have one) and being as transparent with their sources as possible. But people do get things wrong, and I'd say more often than not it's just an honest screw-up (especially for a complicated story) and in my experience the really egregious ones come where a massive story gets sent to the production desk minutes before its deadline. I remember recently having to cut about 30 lines in a 1200-word story and write up captions, a headline and a write-off and having about seven minutes to do all that. I've never actually asked my boss what happens if one of these papers I work on atm misses a deadline, but I've worked at places where the paper gets fined $10,000 by the printing press for missing their deadline. Time pressures are real, and it's not a coincidence that more often than not an organisation that's first with a breaking story ends up issuing a correction not long after.
*Most importantly, when dealing with the media people tend to ask about an organisation's bias. This is the wrong way to go about it. Point blank. The key aspect to a good organisation is detachment. Not going out of your way to look for certain stories, not burying others that you don't like, not making yourself part of the story. This is why I have quite literally zero respect for Breitbart or the Huffington Post - they're not journalists, they're demagogues. This is why I think the obsession against the NYT, WaPo and WSJ is misplaced, because they tend to cover stories that aren't great for their "side" (the Al Franken allegations were very prominent in the NYT, for example, and it's never shied away from covering Benghazi developments where there are actually developments to report) - feel free to ignore the editorials though. All three cable channels are absolutely godawful although all for different reasons, and all three have their exceptions, which complicates things further. But I've made my views clear about 24-hour news channels of all broadcasting persuasions and how they're incentivised to simply be a torrent of similar opinions that are designed to make people feel angry without any actual substantial facts (I can point out a few examples in this thread). That constitutes a market failure in my books - Fox News being the first to strike it big with its business model was one of the worst things that could happen to democracy, and the rise of social media bubbles has poisoned the well to a similar extent. Again I'm not sure what the answer is because sensationalism and partisan hype clearly sell (and Donald Trump's relationship with the media is way more complex than anyone gives it credit for but all he's done is do a masterful job of hijacking that very process) and humans are preternaturally lazy when it comes to dealing with facts. It's interesting though that a lot of respected news brands around the world seem to operate off a hybrid model where they get state funding but editorial independence.
Trust is one of the most underrated currencies in all sectors of the digital economy and sadly I think that's being lost for good in the media and politics as it drowns in a muddy puddle of perceived bias, organisational bias, general media headwinds, sensationalism and a lot of toxic players poisoning the well for everyone else and that leading to even more poisoned wells.
Anyway, rant over. Feel free to ignore all of that, and all I'm sure nearly everyone will. But it's an issue where I have experience in both sides of the coalface and it's an extremely broad issue I've watched play out with a lot of interest especially over the past couple of years. And despite all the thoughts I have on the issue it's one I've generally kept quiet on because I knew I'd end up with a post like that and that's only scratching the surface of several prongs of the same issue. Anyway, I'm about to hit the road and go on holiday so I'm not sure why I spent all this time on that post - I've probably kept myself bottled up too long.
