The problem is the increasing review burden - with LLMs it is possible to create superficially valid looking (but potentially incorrect) code without much effort, which will still take a lot of effort to review. So outright rejecting code that can identified as LLM-generated at a glance, is a rough filter to remove the lowest effort PRs.

Over time this might not be enough, though, so I suspect we will see default deny policies popping up soon enough.