Maintaining data integrity is probably fairly important to them. By allowing a separate service which is used by a significant portion of the students, the system is open to serious exploitation. Professors can get black-balled by disgruntled system administrators or hackers, just by screwing with the numbers.
Yale owning both the data and the interface is understandably important to them, though this might be an opportunity for the creators to work with the administration to reform the current system.
Blackmail is illegal, and intentionally screwing with ratings is almost certainly libel. Both of those hypothetical can be dealt with by the law already; I don' think there is much sense in warping copyright law to protect you from the possibility of those things happening.
Yale owning both the data and the interface is understandably important to them, though this might be an opportunity for the creators to work with the administration to reform the current system.