Nope

Do whatever you feel like doing.

Take buy european as a best practises not as law from god, you could end up renaming lakes and shit.

I never really thought about it but there are patronage and crowdfunding options from Europe such as Liberapay.

Maybe, but perhaps contact them and say you'd prefer to suppport another way... they just might not be aware

It's a bit like credit cards, there is seldom alternative.

I take it practically. I use android instead of sailfish mobile device, at work need to use windows, and I use credit cards. So I use also US based platforms to support content creators when there is no alternative.

Message them to use european platforms and reward the ones that do.

Is the content creator European? Does the US-based payment processor take fees? Those are the questions I'd ask. If you don't like the answers, I'd reach out directly to the content creator and ask if they'll take a donation via a more acceptable payment system. Though, be warned, that's kind of a red flag. "Hey, sign up for this thing you aren't using, link your bank account to it, and I will send you money." I'd probably ignore that message. So, maybe if there are a few you use, ask them if they take any of those.