Gamers aren't exactly known for being hugely inclusive of people who aren't cishet males. A game based on an IP by an outspoken transphobe is the perfect thing to use for a political point towards gamers. It allows people to say, "ok, if you're actually an ally just do not play this game for a few weeks" which in turn exposes people who call themselves allies just because they won't directly call someone a slur. If you refuse to do the absolute easiest task to show support, it's not fair to call yourself an ally. The original point wasn't to stick it to JKR, it was to prove self claimed allies don't actually do shit.

With anything on the internet, it got too big and people didn't understand the point. They just saw "boycott Harry Potter" and assumed it's about monetary incentive. Thus it turned to "all people who play the game are transphobes and we will harass them" and then spoiling the game out of pettiness.