I think this is like predatory lending laws - you can be forced to give someone a loan at a reasonable price, even if you can convince them to pay way more. The same argument also works for voluntary slavery - I think people have the right to say "even if it's initially consensual, it still seems like an abusive business model that we shouldn't allow"

You aren't forced to give someone a loan at a capped price, you can simply refuse to loan out your money. The government does not compel you to loan, so it is not analogous.

I get the logic of rendering certain voluntary transactions illegal. Child labor (questionably voluntary) & minimum wage laws come to mind. This is distinct because those laws don't require companies to hire people at minimum wage.

Yeah good clarification sorry, the price cap is the compelled part, not the original loan decision

Only if part of that transaction involves non-material things such as personal data. If you accept actual payments in money, you are welcome to refuse service to anyone. The solution is easy and straightforward.

Nope, Facebook is free not to provide those services.

They just aren't allowed to provide them by spying on you.