For many students, perceived campus strictness is a result of the administration's tendency toward hypocrisy. Why all the rhetoric about "independent women" if you're not going to let legal women drink in their own living rooms? Why assume students are responsible enough to take on complicated, important ideas and then make them sign up if they want to have a party? Why treat students like adults in the classroom but like children outside of it? Sometimes campus police or res staff will pretend they don't know what's going on, but more often, any violations of the extremely strict alcohol policy will get you a date to defend yourself before your peers and faculty on the Alcohol Hearing Board.
Of couse, it is a liability issue—if Wellesley were to get slapped with a serious lawsuit, it would affect their ability to educate. And obviously the law says you must be 21 to drink. But students understand all this and still see more leniency and less "parenting" at similar schools. A word to prospies: You're not going to win this battle, so if you like the idea of keg parties, having some beers on the dorm lawn, or even a quiet dinner with wine in the dorm kitchen, reconsider Wellesley.