Even if donations are not the main point here, "donations welcome" would still not sound appropriate to me for the reason mentioned, so I'm not sure you get my point.
Saying that something is "welcome" is almost like saying it's allowed.
You're not kindly permitting people to give donations, or telling them "Don't worry, we have no problem if you really want to donate, because we are lovely, welcoming people".
Even if they are not expected to give a donation, you'd still be grateful, right? Not magnanimously
agreeing to accept donations?
Yes, there is a difference between "welcome" and "welcomed", as explained.