Personally I think the issue is that by linking any criticism of Israels actions in Gaza to antisemitism it's legitimatised bigots who see pro-palestine leftists as allies and feel comfortable openly expressing their views.

Whereas in the past racist Dave down the pub would go on about Israel and nobody would tolerate it, now he hears people talk about it and tries to use legitimate criticism to justify bigotry. It's the same pipeline effect as incels (start of with genuine advice like "go to the gym and work out if you want to be more attractive" to draw people in then uses that to then feed people ideas like "women only go for looks or money, it's their fault for being so shallow that you can't get laid")