Customers have to abide by terms of service.
I pretty much think it is the other way around, that terms of service can not infringe upon individual rights arbitrarily.
Such as discriminating against political opinions one may not agree with.
Because that not only prevents truth from being revealed, but also allows harmful discrimination based on race, religion, national origin, etc.
If a baker refuses to bake a cake for your gay wedding, that is not just inefficient in that you have to try others until someone will, but also is hurtful, humiliating, etc.
In the US we have the 1965 Civil Rights Act that lists a half dozen groups historically discriminated against, who are listed as now specifically protected.
But it is amazing how many lawyers then do not at all understand law, and think ONLY those half dozen listed are protected.
They do not seem to realize ALL discrimination is inherently illegal, and that these listed ones can not possibly be the only ones, because that would specifically violate the 14th amendment Equal Protection clause.
It is illegal to list and protect ONLY a select group.
It has to be all of nothing.
When you explicitly protect some, then you also have to protect everyone else.