If you want to host public projects, you will always have some responsibility to the public. Similar to how hardware makers shouldn't be making hard to repair their hardware.
I disagree with this completely.
If you have the right to turn off PRs, any company out there also has the right to make thing that are hard to repair. I don't want to say anyone who agrees with you on this thread complaining about Google or some other company shutting down your accounts with no explanation either.
Joke's on you; I already do shut down every PR automatically on my projects with the repo-lockdown bot: https://github.com/marketplace/actions/repo-lockdown
Making my code public at all is what costs me nothing. I am already writing it. I am already versioning it with Git. Giving you access to it is either a no-op or is some amount of public good. It can never be a negative. This is what Free Software is. Read Stallman: https://www.gnu.org/philosophy/free-sw.html#four-freedoms