I'm surprised they didn't enforce this earlier on but are suddenly so adamant about it after killing the program.
This really makes me think the reason for this that they're phasing out physical games. Sad really.
To me, it almost seems like they are gearing up to discontinue the program entirely later this year. First, stop customers from buying or renewing GCU. Next, use a "loophole" to get rid of a bunch of still active GCU accounts to lower the total number of users. After the dust has settled from that announce they will be ending the program and issuing prorated credits/refunds to those that remain. By getting rid of a bunch of accounts that had violated the GCU TOS they will pay out less money/credits when they ultimately discontinue the program.
To me the writing is on the wall with this and it may happen sooner than you think.