I've dealt with similar-ish situations before and would suggest that you consider (I am not recommending, merely suggesting) adding a config option -

    thanks_sponsors false

then people who really hate that header can disable it but it's clear what they're doing with the software they're paying no money for. If people complain about that well, that's their issue IMO. And they have to be explicit about how they're behaving.

This has worked well for me in the past. If you'd be interested chatting about how/why reply and I'll drop you an email.