OK, two things:
(1) It's not that drinking in letters is wrong. I'm 21, I drink occasionally, and there's nothing wrong with that legally or morally. I think sometimes orgs put limits on what activities you can participate in (when wearing letters) for the same reason you have to be wearing badge attire when you're wearing your pin -- because it just shows you have THAT much respect for your letters.
(2) Like 33girl was saying, we all know if you're an XYZ or not. Or at least that girl in the back knows, and she's telling that guy who's telling his girlfriend who's..... etc. Which is why some organizations (is this only in the South? anyone want to jump in?) put OTHER requirements on their letters. On my campus (in southern Virginia) 4 of the 5 sororities have rules about their members dancing on bars. NOT allowed. Period. In letters, out of letters. It's a hobag clause.
