Yes. Why shouldn’t it be? I genuinely don’t understand your argument.

It's up to the registrar to make the rules. If nic.io is fine with people outside of that territory to use their tld then it's okay.

Some TLDs do indeed have stronger requirements. I remember that for a long time getting a .fr was a bit of a pain because you had to prove that you were a french citizen or a french company IIRC.