There is a difference between private people voting with their wallets and encouraging a mob to pressure a company to withdraw advertising from a company that they oppose politically.
It's not "encouraging a mob" if someone say "Hey, these companies are advertising with bigots and racists, let's do a boycott to show them that we will not tolerate this".
What you are saying is that they should "shut up and dribble"? Stay quiet with their heads low while they are being constantly attacked?
Calling for a boycott when you don't agree with someone is pretty far from "encouraging a mob" as you put.