Obviously crew liquids in any quantity are inherently unable to be used for nefarious intent, unlike non-crew liquids.
This is one of those ridiculous rule applications that drives me nuts:
TSA/DHS knows for the most part it can handle backlash from the general public because any objections will be disorganized and "manageable" (although I really hope that it has crossed the line and this will change).
Crews, on the other hand, are one area where it has to be at least somewhat careful...because they really don't want to get ALPA, APA, and the FA unions collectively on their case in a major way, because they have the wherewithal to actually be troublesome if they so desire.
The liquid restrictions would have had a major impact on crew...who carry everything on and probably don't want to have to buy anything they drink at crazy airside prices (at least anything they want to drink that their carrier doesn't stock on board).
The solution? Make them exempt, of course. If it doesn't apply to them, then they obviously won't make a stink about it.
If asked, I am sure the official TSA response would say something about how "low risk" the crew are. If that is the case, however, why make them go through security in the first place? Either they are a risk, which means they should be screened to the exact same standards as everyone else - with the same prohibited items, or they are not, which means they get a free pass.