No one actually believes Jim Acosta did anything wrong.
This is one of those situations where conservatives collectively pretend to believe something they don’t believe.
Pretending to believe something they don’t believe serves multiple purposes:
One, it signals tribal loyalty. Being willing to say blatantly false things shows you put tribe over truth, and that is a critical loyalty test, as anyone who has studied cults can tell you.
Two, they’re trolling. “Triggering the libs” is the meats and potatoes of their political ideology these days (read my book, Troll Nation). Telling blatant lies and watching liberals make themselves crazy insisting on “facts” and “evidence” makes them laugh in delight.
Three, they are trying to push the idea that violence against women isn’t a real problem, but something people only pretend to care about to score points.
How better to seed that idea than only pretending to care to score points?
Fourth, it's about reestablishing Trump's narrative that the media are the "enemy" to be defeated instead of a Fourth Estate necessary to keep democracy healthy. This helps, in turn, fuel the already strong message that democracy is wrong and authoritarianism is right.
Fifth, it's a straight up distraction. Trump wants everyone "debating" a completely obvious video, instead of talking about how he's trying to conceal what is almost certainly very serious crimes by interfering with the Mueller investigation.