It should only be to give a reigning champ a chance to get their belts back to avoid randoms like Ruiz, Rahman etc from becoming champ only to lose it to someone else. Only champs should get rematch clauses.
In regards to Bud/Spence, it's in the contract that if the loser wants it, the winner must grant him a rematch, so there's nothing to complain about. However, that same contract says the winner gets to pick which weight class it happens, so if Bud wants it at 147...