Standard stuff in the business world. You go to an agency who places you with one of their clients eg: big airline. The agency gets a cut of your wages/salary etc. After a while you decide you want to be permanent and have more job security (I use that term loosely in the current environment
). The agency built up the relationship with the client and introduced you and is about to lose out on cash. Hence why they have these sorts of clauses. Works in the same way if the airlines accountants one day decide to reduce temporary contract expenses by transferring everyone to permanent.
As stated above you either buy yourself out or get the airline to if you want to go onto a permanent contract.