I've gone ahead and unprotected these 3 questions now; as you point out, there's no possible issue that can arise from unprotecting questions that have been permanently locked. (I think all types of post locks other than the comment-only lock do prevent anyone from posting an answer on the question.)
I also agree that questions that are permanently locked, with no possibility of being unlocked, should be unprotected. Unfortunately, sometimes it can be tricky to know whether a post will remain locked permanently (e.g. during a developing situation). Other times, though, as in the case of historical locks (e.g. as a result of a change in policy causing a type of question to no longer be allowed on RPG.SE), there is indeed no benefit to keeping them protected, as they'll presumably never be unlocked - so there are no spammy/bad answers that we need to protect them from.
Note, however, that the same may not be true of temporarily locked questions. Questions can be locked for shorter periods of time ranging from an hour to a week. In those cases, we should not immediately jump to unprotect the question. (On that note, there was a past incident in which I (as a moderator) could see the duration of a temporary lock but others mentioned that they couldn't see that the lock wasn't permanent - but that issue seems to have been fixed with the rollout of the "new" post notices.)