The Yale administration is notoriously lenient towards most student activities, especially alcohol. The university tends to value safety and moderation above punishment or strictness, and the Yale police and staff are more likely to warn students about certain behaviors than write them up. A common example of this policy occurs during fire code inspections. Students are technically "not allowed" to have microwaves, but it is assumed that most suites will own one. All students have to do to pass fire expection is cover their microwave with a towel. This relationship between Yale authority and its students is typical in many areas, especially alcohol. As long as you don't reach extremes, the administration won't bother you.
Beginning at freshmen orientation, Yale makes it clear to students that the University expects underage drinking to occur on campus. Students are encouraged to consume in moderation, in safety, and to look out for one another if they choose to drink. Yalies will never get in trouble for going to Yale Health Services after drinking too much. Students are warned, however, that if their behavior or condition is severe enough to involve the New Haven Police or Hospital, they will face consequences.
Yale's policy of "safety before punishment" receives mixed reviews among students. Many students enjoy the lax atmosphere and take full advantage of it. Others feel that the system reflects a lack of care for the students and leads to too many dangerous situations. The University is very strict on certain issues, however. Sexual harassment and/or rape (under any circumstances) is not tolerated, nor are hard drugs and narcotics.
Essentially, Yale attempts to walk the line between student safety and free choice. The system has its pros and cons but has, in general, supported a positive atmosphere on campus and promoted the safety of its students.