Short answer: Linux does it because they can.
I understand that @JBits' comment might imply "repositories that accept AI-assisted PRs are untrustworthy," but that's actually the inverse of what's been said, and it is not logically equivalent.
AI does write high-quality code nowadays so most repositories here might even use AI internally themselves. They banned it because they simply don't have enough capacity to ensure that quality from others.
Imagine that earlier you can just reject bad PRs from poorly written descriptions alone. Now you have more volume that passed the first gate, and second, and third. You cannot distinguish them from contributions where the author spent time into making sure it fits into the repository and actually adds a global value.
This also makes rejection harder as explaining counter-intuitive things (why good quality is bad sometimes) is naturally difficult. The idea is coherently written. The code looks beautifully written. The tests passed. What stops you from merging?
Linux has all the resources (and stakes!) it needs to welcome those contributions.
