The intention of this was (I presume) preventing players from buying slots for which they don't have aircraft.
The unintended consequence of this rule is making re-scheduling (fleet transition) a nightmare.
Ideally, would it be possible to trigger this only when player does not have enough aircraft without schedules? I have 100+ aircraft without schedule, and I am being penalized for having unassigned slots for no more than 25 aircraft.
2nd best option would be to quantify this for the player such as:
"You currently have X unassigned slots. Your airline is only allowed to have Y unassigned slots"