As with anything on the Internet, peoples perceptions play a large part.
Vampy has a perception that leyshon will put in a decent shift at NATS for a few years before heading off to pastures new. I don't think anyone would have a problem with that case. The time, money, and resource spent on the young chaps career would be paid back before he left.
Others like me get the perecption that the gent would be out the door as soon as he could. So we all spend a year or more of our time and energy filling up a shortfall only for the ATCO to go again as soon as we have given him the means to do so. Given the choice, I think most of us would rather devote our resources to people who are going to contribute to easing the shortages we already have, people who want to be with NATS for a good few years developing themselves before seeking out their ambition elsewhere.
I'd propose a 5 year sliding training bond to prevent this kind of thing happening. It's common place in many airlines.