You sort of have a point; the letter explicitly calls out that they're reconsidering providing this data at all.
But... given an adversary's policy of "we will provide this data if and only if it goes unused", there's no reason to give that policy any weight in deciding whether to use the data. If you decide to use it, the policy will kick in and you can't. If you decide not to use it, it's not relevant that it remains technically "available" for other people to also not use. That's not a reason to consider what Yale wants, it's a reason to slam them for being weasels.
If the university no longer provides the data this seems like a good opportunity for the application developers to manage that data themselves. They already have the students' course information so they could email them at the end of term and get their own evaluations -- removing Yale from the equation entirely.
Also worth pointing out that it's in Yale's interest to collect and provide the data. If they collect it, they can ensure a more representative sample size, even if they can't ensure how that data is ultimately presented. If students start using a third party service, not only does Yale still not have control over presentation, the data collected will probably make the faculty look worse by virtue of the fact that disgruntled students are more likely to respond than happy students.
But... given an adversary's policy of "we will provide this data if and only if it goes unused", there's no reason to give that policy any weight in deciding whether to use the data. If you decide to use it, the policy will kick in and you can't. If you decide not to use it, it's not relevant that it remains technically "available" for other people to also not use. That's not a reason to consider what Yale wants, it's a reason to slam them for being weasels.