"Jack
Well obviously that would be a concern but on the whole it would benefit the game. You wouldn't have to release staff for silly prices although you would be paid for a staff member being bought out of contract.
And anyway, people who do hijack staff would get a reputation and so wouldn't be top of most people's selling list!
Imagine it the other way around from what you proposed. You suggested this, I'm guessing, because you want to hire someone else's driver or staff. What if you had hired a driver or staff member who you had either lucked into or had spent a lot of time finding or developing. Then, the day before your next race, someone else hires them out from under you.
It may work that way sometimes in the real world, but I think it's an element we can do without in the game, personally.