7.5k is not that significant for most rated joiners as it is likely they are joining for their base of preference, making it unlikely that they'll leave any time soon.
However, it does prevent the odd curve-ball situation where a rate new joiner suddenly gets invited back by their previous employer with a carrot on offer (such as base/part-time etc) meaning Jet2 have wasted the few thousand invested in induction and OCC etc.
Not saying I'm a fan but can see their point of view, and still a lot fairer than what most operators are offering when you bear in mind the contracts.