There are all sorts of things about college athletics and restrictions on transferring that are unfair towards the students.
This is just one of them.
USU looks at BYU as a competitor and so has little desire to help them out.
Perhaps their point of view is that they are trying to scare away other current USU athletes from transferring. They are trying to make their own players think that transferring is such a hassle that they might as well stick around and finish out at USU.
It does seem like USU will eventually get a bad reputation out of this, but maybe USU thinks this dramatic public situation will eventually blow over.