Again, availability bias. The people screaming the loudest about this are also screaming the loudest about other Left/Liberal issues. The normal left-leaning folks don't get heard. It's pretty natural to conclude that this group represents the whole.
I'm also convinced that the screamers aren't really interested in stopping black people from being offended (let alone actually harassed/murdered). I think they're much more interested in getting recognition for fighting the fight.
I think you're right. I'm an independent and I'm guilty of doing this to right and left leaning folk, but I've done it because I believe that the people screaming will only listen to people within their ideological interests. It was my way of making them accountable for their compatriots that are loud enough for me to notice. I've also stopped doing this because it's not really effective. People don't feel like they should be responsible for outliers and I somewhat agree, but don't know how to solve the problem of vocal trouble makers.
I'm also convinced that the screamers aren't really interested in stopping black people from being offended (let alone actually harassed/murdered). I think they're much more interested in getting recognition for fighting the fight.