Also: if you are not paying the service provider for the service, you are not their customer - you are their product.
Having said that, you're right to be suspicious of commercial services, even that you pay for. Someone can found a startup with a strong commitment to customer privacy and the best of intentions, but a few acquisitions or near bankruptcies later, those commitments will go out the window.
The small chance that they might go out of their way to not sell premium users data doesn't seem worth much.