I would guess it's because they don't want to penalize teams for developing players at the highest level of competition they deserve to play at during critical development years while the team isn't benefitting from that player playing on their roster. They're technically a "junior" aged player, so unless the team is benefitting from that player's play directly, the contract slides. Not necessarily a "are they pro or not" but rather a "are you still developing the player at a level lower than the NHL during their junior years" deal.