"post about it in a local artist community" is not exactly "talk about it with their friends."

It's bullying.

And if the shop owner is Jewish, it would be anti-semitism:

https://en.wikipedia.org/wiki/Anti-BDS_laws

I consider a boycott threat a non-violent threat; legal but an exercise of power that has moral dimensions.

It's informing people that there is something a business is doing that they might not approve of and they might want to change their behaviour accordingly.

Imagine if it said "post about it on Hacker News". That's not bullying