In the former case, disentangling AI-edits from human edits could tie a project up in legal proceedings for years and projects don't have any funding to fight a copyright suit. Specifically, code that is AI-generated and subsequently modified or incorporated in the rest of the code would raise the question of whether subsequent human edits were non-fair-use derivative works.
In the latter case the license restrictions no longer apply to portions of the codebase raising similar issues from derived code; a project that is only 98% OSS/FS licensed suddenly has much less leverage in takedowns to companies abusing the license terms; having to prove that infringers are definitely using the human-generated and licensed code.
Proprietary software is only mildly harmed in either case; it would require speculative copyright owners to disassemble their binaries and try to make the case that AI-generated code infringed without being able to see the codebase itself. And plenty of proprietary software has public domain code in it already.