The only way they can enforce this, it would seem, is by parking a ranger at the base of the cables pretty much all day, and checking for permits, since the trail is the same as used for Clouds Rest and other destinations. So it the ranger going to camp out up there all summer or hike up each morning? Doesn't seem to make a lot of sense.
This may sound stupid, but how about putting in another lane? The traffic jams seemed to result from people going both directions and some folks afraid to let go of one side of the cable. Maybe they need an up lane and a down one.