Because people were taking advantage of the system, as I'm sure you know Zooker. They'd sign the mobility agreement in full knowledge that they would only stay in NATS if they got unit X, complete the rating course, be sent to anywhere other than X, and so leave the company to work outside of NATS near to unit X, so presenting that unit with a 'free' trainee, who may have even approached validation at their original NATS unit to make themselves more employable.
NATS decided to take steps to prevent that happening. Can't blame them.