Not 100% sure about the exact US regulation, but in Canada (and we're very similar in a lot of cases), you would not be permitted to drink it inside the checkpoint. There are a couple reasons for this. First of all, it slows everybody else down behind you because an SO has to waste time watching you do it and secondly you're technically already in the restricted area. It's your own fault if you didn't think to empty it out beforehand anyway, so suck it up and do what you have to do instead of blaming the poor guy who's just doing what's required of him by government regulations.