Well...some protests do have certain rules. Union strikes are one such example. The union can picket all they want, but it is illegal (and will hurt them in negotiations) if they refuse scabs anyone entry into a facility.

Sit-Ins are kind of the same way. The police can and should be allowed to arrest the protesters. Pressing charges, that's a different matter, as the protesters typically don't do anything illegal. Certainly they're not barring entry on a sidewalk. Forced eviction and arrests are pretty common outcomes of sit-ins.

Was this instance complete bullshit and the police stepping over the line? Yes.