This answer is accurate but no one seems to realise it. Under standard contracts users waive their rights including privacy rights under GDPR. Their are no user agreements anymore, only contracts.
You can absolutely choose to share your data with an US company, but you can also withdraw consent five seconds later, making it illegal for them to further process the data. The point is, that it is your personal data, and only you get to say what can and can't be done with it. That right can't be signed away.
European laws have a different approach regarding most individual rights. They are socially defined and distinguished from ownership.
Your body for instance is probably better protected in terms of chemical additives, healthcare and now pregnancy choices, but you can't easily sell body parts or be a surrogate.
If a service operator decides they don't want you using their service, they don't have to let you use it. That only gets murky around protected characteristics (race, sex, gender, orientation, etc) which "privacy" is not.
They could make access contingent on being allowed to process your data.