“What exactly would the cops do that was a violation of (somebody’s) first amendment right?”
Preventing people from assembling.... clear violation of the first amendment. That is what people seem to be retroactively demanding that the cops should have done.
“I’ve suggested the cops arrest a few of the ringleaders either on open warrants or some pretense.”
Which probably wouldn’t have stopped the rest of them from showing up, so people would still be complaining and trying to find a way to pin this criminal-initiated violence on the cops.
So, Stroman's cover story is that he's upset the proprietor wouldn't facilitate prevention of assembly?
Your theory is poppycock. The government wouldn't have been preventing assembly, per se, it would have been closing one venue, at most; and by enlisting the proprietor, there is no violation, period. There wouldn't be "government action."
See, for example, Mall of America seeking a TRO against Black Lives Matter.