I agree with you that paying to get training should not be done but I don't think that the market is so good for the moment. I also agree that a lot of airlines in UK are hiring but you have to be type rated or willing to pay the rating. According to me the Jet2 CTRS is just a kind of training bond. You have to pay the rating but they pay you back during 3 years, fair enough for me. They just want to avoid you to get the rating for free and then escape after 500 hours for greener pastures. Sound quite logical on an economical point of view. Once you are in the company and you change of type/seat I agree at 200% that you shouldn't pay. It's a 2 ways street, you did the first step by paying your initial rating, they now to have to take the risk to pay for your upgrade.
Regards