This behavior baffles me too. Last summer after visiting the Sears (yeah I know) Tower during a long layover at ORD I was a couple people away from the podium the three striper who was training a new TDC politely said something along the lines of "Sir you can't take that through the checkpoint." and motioning to the camelbak bottle I had clipped to my backpack. At this point I mentally face palmed since I had specifically sought out a restroom to dump it out in but forgot to after my belt broke. She then said I could dump it in the big trash can between the WTMD and NoS or just drink it. She also pointed out where I could find one of the water fountains nearby that had second spout for filling bottles which I thanked her for as well as not just taking my bottle. Since there was only a few ounces left I just drank it taking just enough time to let the line for the NoS to get a few people deep before strolling over to the WTMD. Now if all the TSA clerks could be as polite and professional as she was things would be a lot better for everyone.