I hate how many people are like, "But they're sponsored by gambling sites!" So, two things here. First off, players in a league betting on games of that league can basically be looked at like insider trading. Making money deals that potentially screw over others based on non-public information. Secondly, no league is going to want it to look like players are influencing the outcome of games for money. That type of scandal could cost the NFL millions. Even a small shred of evidence can do a lot of damage. And now that they're sponsored by gambling organizations they have to be extra careful with the image, and more proactive in shutting it down before we end up with the NFL's version of Pete Rose.