Sure, there are sites that mistreat names in ways you describe, but that does not make it correct.

> I do in fact want a properly functioning service to use my name in a vocative case. ... I also don't want to know to specify the correct version myself, as it's trivially derivable through established rules of the languages.

There would be nothing to discuss if this was trivial.

> Not doing that is plainly grammatically wrong and borderline impolite.

Do you know what's more than borderline impolite? Getting someone's name wrong, or even claiming that their legal name is invalid and thereby making it impossible for them to sign up.

If getting a name right and using a grammatical form are mutually exclusive, there is no argument to be had about which to prioritize.