> I would prefer never to see somebody else's copyrighted solution to an issue
You are doomed, then. They can always sent a (unrequested) email to you, Cc'd to a public mailing list. The PR interface is just another equivalent way for people to send messages to you, like email.
Somebody's really going to have to go out of their way to email a patch — they might even read the `CONTRIBUTING` document which says "Thanks but no!".
The Github Pull Request interface is an attractive nuisance, like an unfenced swimming pool. You can't keep people out with warning signs, they won't read them. Put up a fence, though, and even the obtuse will get discouraged.
In order to keep the IP for a project pristine (for the sake of downstream users), I would prefer never to see somebody else's copyrighted solution to a problem. If I see their contribution but I don't want to accept it, I'm forced to come up with a new creative solution that a court would (maybe, hopefully) consider original.
This is especially important for CC0/public-domain projects, where many contributors will not fully understand the implications of contributing and may not be in a position to release code into the public domain.