Not sure what blocking re-employment has to do with leaks. If anything driving people to other companies is likely to cause MORE leaks.
This is almost certainly about eliminating the risk of contingent workforce being classified as employees. My own employer does the same thing, though it does not bar long-term relationships as long as the company doesn't interview individual workers. That is, if we hire Fred to help out with something, then Fred is gone in two years and must take a break. On the other hand, if we hire Acme janitorial to clean our trash and they send over Fred then he can work for years, but we don't get a veto on who they send/etc.
I have mixed feelings. On one hand it does make things harder on those who end up having to move on. On the other hand, before the policy we used to have a LOT of people who would be dragged along in a contract position with the elusive promise of a hire that would take years to happen. The policy forces managers to act if they don't want to lose somebody.