I think this is more about being consistent than anything else. Polk supposedly didn't allow drinking either- we all know how that went. And per "The Book" a player that is caught drinking is to be kicked off of the team.
Plus, drinking could be a big problem with the team. I have seen at least one person say that part of the reason that we played crappy was because they were out partying all night. I don't have a problem with them drinking, but if they can't handle drinking, and it affects their play, then something needs to be done about it. It's not any real secret that drinking and MSU baseball players go hand in hand, despite what Polk wanted people to believe. It's a tradition that dates back who knows how long.
Just to be clear- I don't think that the players drinking and partying too much was the only factor in their crappy play. In fact I believe it was maybe 1% fo the reason, if that.