They like money. Controversial is not illegal. Slander is. If I purchase billboard space and spread defamation that will be problematic. The ISP could always take me to court but they would very likely lose provided I can prove I am telling the truth.
I know some (all?) EU advertisers deny creatives based on optics i.e. "our name and logo is on the billboard frame, we don't wanna get associated with topic X".