People are overthinking this. When someone's contract is running down and you want to keep them, you offer a new contract - otherwise they likely leave.
As a club, we tend not to stand in the way of people who want to leave - players or managers. (Most businesses are the same way, as it doesn't make sense to keep someone who is unhappy, and slavery has been illegal for quite some time).
The compensation thing is a bit of a red herring. If Robinson just walked out today, breaking his contract, there's not much we could do except sue him personally - and I'd be surprised if we would get a year's salary from a case like that as it's not like we paid him the money up front.
As far as Preston are concerned, if they get promoted and finish bottom of the EPL, they earn about £100 million then receive around another £42 million in parachute payments. Of course, if they stay in the EPL they earn a lot more.
So that £3 million payment is 2% of 1 season's minimum potential earnings should Alex Neil succeed. If we asked clubs to pay 2% of one season's of Motherwell's potential earnings should Robinson succeed, we'd probably get a lot less than we are asking now.