I guess my point is that the MMEL just tells you how to deal with certain lavatory equipment failures. It doesn't tell us whether that was actually the case for the OP. In fact, it implies it was not given the apparent lack of placards.

By posting this, I think you meant to imply that unlocking a lavatory is a punishable violation of some kind. But, I think it tends to prove the opposite, in fact. Without a placard, the lavatory is almost certainly operative and the OP's suspicion that the crew is locking the lavatories for their own convenience is quite likely the case. Unlocking such a lavatory doesn't seem to be against an particular rule. Of course, crewmembers are likely to get the benefit of the doubt if they decide to make a "federal case" out of it. But, in doing so, it would probably come out that they were locking the door improperly and would likely be punished themselves.