This complaint is in my opinion rather specific to GitHub (and could thus likely be solved by switching to a more repository provider, or hosting the repository oneself), but not being able to disable PRs at your own repositories is like having a blog hosted by a provider that does not offer the option to disable blog comments.
Eh, sounds like a really minor annoyance. It's clear that the PRs aren't your own content, unlike comments which you're sorta responsible for moderating or else you'll often end up with vile stuff all over your family-friendly blog post.
Cause PRs aren't presented very upfront to visitors and are far less frequently abused. A lot of the open source world relies purely on goodwill, just like the author says.
Also, blogs are on your own domain name usually, which at least gives the illusion that it's your own website rather than just your little tenancy on a blog platform. Some platforms don't give you a domain technically but a / instead, like FB Pages and Reddit, and they explicitly assign you the responsibility of moderating your own page. If their own moderators have to step in, it often leads to deletion.
Liability disclaimers are a legal grey area. Even the US pulls aside some developers at customs because of their work on OSS projects. Imagine what some of the least tolerant countries do.
