Basically, NATS is going to spend a fortune on you, with no guarantee that you will ever earn the company a penny in return. It's likely that your training will cost the company a large fraction of a million quid once you're valid.
So your side of the bargain is that you have to let NATS choose where they send you and what discipline you do. If it's still the same as when I was at college, you do get some input in the process, but ultimately NATS has requirements to fill and so management gets the ultimate say.
It's harsh, but ultimately fair when you think about it in these terms.
Edit> I can't believe how many times I managed to use the word "ultimate" in that post
