There's no implication that they would all be receiving the same cure, though. They just happen to be in the same facility.
And as for them being the only incurably insane individuals in the universe... I'd rather argue that they are the only criminals the UFP both
1) caught and
2) subjected to the usual routines of helping the criminally ill (that is, all criminals),
and unlike the billions of other criminals throughout the history
of the criminal curing system (out of the trillions upon trillions of individuals forming the UFP, most of whom are not criminally insane), they currently represent the lot that has immunity to this usual cure.