Because in this case some people are going to have to wait up to 4 years to realise a bonus that was awarded on the basis of helping the company succeed in 2017/18. If you leave in the next 4 years then no bonus for you despite you being part of the contributing force to the success. This just seems cynical and unnecessary, I for one have just had the last glimmer of good will kicked out of me, they can get F*&ked.