Actual spam is typically hidden, rather than being locked, from what I understand.
If people unlock threads that were locked by mods, it would obviously warrant punishment. That said, something being against the rules does not necessarily prevent it from happening, nor does threat of punishment. The only real way to prevent it would be to make them incapable.
Even so, it is important to keep in mind that threads being locked by mods is actually quite a rare occurrence. The situations in which a thread will be locked by a moderator, and the member chooses to unlock it of their own volition would not happen often. Making a note in the mod forum when a thread is locked would not be difficult (I am not certain why Catterjune seems to think there would need to be PMs, that is just ridiculous) and if the thread was unlocked it would be simple enough to work out.
Thus said, it really boils down to a conflict between the benefits (which have been stated multiple times, and each have their own merit) and the risk of just how much damage someone can do after unlocking their own thread before a mod realizes what is going on. With a vigilant staff, I sincerely doubt much of issue could really happen, but, put simply, vigilant isn't the first word I would use to describe the staff as-is.
All in all, it really is up to the staff. It is their judgment call on whether the risks in this are real, or whether it is just a minor amount of additional bookkeeping so that members can have more freedom in how they use the forum.