Ref T13289. This tightens up a couple of corner cases around locked threads.
Locking is primarily motivated by two use cases: stopping nonproductive conversations on open source installs (similar to GitHub's feature); and freezing object state for audit/record-keeping purposes.
Currently, you can edit or remove comments on a locked thread, but neither use case is well-served by allowing this. Require "CAN_INTERACT" to edit or remove a comment.
Administrators can still remove comments from a locked thread to serve "lock a flamewar, then clean it up", since "Remove Comment" on a comment you don't own is fairly unambiguously an administrative action.