Aren’t they totally dependent on the good will of the model providers?

I think it's more likely the other way around. If someone is offering low friction access to your model, you don't want to piss them off.

In what way? They're just an API customer like any other and charge a bit more on top. Providers would have to carve out their usage terms to not allow resell, which does nothing besides lose customers to competitors. If they all did that then you would tap on the FTC's shoulder and suggest they do their job.