It's true that's the case now. But Terry Boot was one of the earliest muggleborns bound over for service, and the contracts were not standardised at that point. It could be that they simply used an agreement that Carrow drafted, and Merlin knows what foul conditions and clauses he might have put into it. He would have wanted total control; that seems to be his modus operandi.

Anyway, it's a good thought and worth pursuing. I'll definitely look into it.