Most social customs are pretty arbitrary, but they exercise surprising power over people's minds. You have to keep that in mind when you go against them. The cops who arrested her probably were unaware of the 1992 state appeals court ruling, given that it is surely not tested very often on the streets of New York. ;-)

So they were no doubt under the impression they were doing "the right thing" in arresting her.

You would never find chimps arresting anyone over something like that.