They can't fine them anything. They can refuse to do deals with them. They can't fine ANYONE that isn't inside the U.S. Well, I suppose they can but it is meaningless.

The EU could say "We are going to add a tax or tarriff on U.S. product X, or increase the price of product X when it goes to the U.S." and that IS within their domain.

That is very different from claiming a U.S. setting its own tax policies is illegal.