This is because a computer that has over 1 gigabyte of traffic in 24 is almost certainly keeping others from connecting. When the student is called to warn about excessive traffic, the student should specify why there was excessive traffic. If the traffic is for educational purposes, there will be no warning. This policy has been enacted because of student complaints that they cannot access the network. Games themselves are fine.
Music is fine.
Only large amounts of traffic are a problem. It doesn't matter if the traffic is music, games, still pictures, movies, etc. As a reality check - if someone uses their Internet connection to download music, they can get 5 full hours a day of MP3 files and still be way under the limit. If one were to do that every day for a week, they'd get over 35 hours of music. Most people would have a hard time indeed listening to 35 NEW hours of music every week.