I agree. These particular women of color are behaving unpleasantly. ;-)

This is clearly a free speech issue in a public space. So they can fuck off.

Someday, I'd like to see a deeply offensive sticker that defies logic. Something like...

White people go back to Africa.

They're guaranteed to find some offended folks for that one.

Avoid defending it by saying,

"I don't know what it's supposed to mean." and...
"It was a gift from my ex." (Refuse to indicate anything about this girlfriend).

Making it a gift from someone else makes it somewhat more difficult to demand removing it.