Maybe when you are level synced any skill that you're not supposed to have is set to a max of potency of 100?
Then your "level 70" rotation would actually be weaker than whatever you're supposed to use at level 15 or whatever, and therefore you'd be actively holding your party back if you insisted on using it anyway. Therefore, why even have it available at that point?

Things are fine the way they are. Again, you want your high level toys, run high level duties or go unsynced.