Originally Posted by ITanNicNic

This'll happen (although probably not the case here) when there's something in the lav that is broken. If it's an item that can be deferred (such as a fire suppression system) they lock the lav for pax use but the crew can still use it.

I dispatched an MD-88 like that, the aft left lav had some fire system INOP and the crew on the inbound took a piece of paper, wrote "Crew use only, sorry" on it in Sharpie and taped it to the door. Myself and a few pax got a laugh out of that, only after it was explained to us why they did it.