I'm not super up with Maori culture (from a cultural standpoint the norm is probably to have the opposing chief/captain come forward to accept but it's a really broad issue and fwiw kapa o pango was specifically designed for the ABs), but the response is just as much a part of the haka itself. It's a challenge and a challenge warrants some kind of response. I wish more teams would do it, but World Rugby rules are pretty restrictive. There have been a few good ones, iirc it was the French some years ago who turned around.
Fwiw a Maori researcher doesn't think it's offensive from a cultural standpoint, nor does the bloke who made the All Blacks' haka what it is today. And the people I've seen over here who agreed with the fine only used the "it's the rules" argument (so no prizes for guessing where they stand on other issues...).
https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=12280290