Earlier quoted context omitted.
How would you structure that bonus if you get a promotion that takes you to 7,501? You would get an impressive pay cut. If you structure it like taxes where only the first 7.5k get doubled then a bunch of people that don't need it will also get free money making the program cost more.
You're right! That would be a major concern and an incredibly serious flaw in such a program. Maybe the description could be parsed slightly differently? Some might read parent's description to be proposing a bonus on W2 income the a limit on the bonus only of $7,500. Thus an income greater than that receives the full bonus. The structural problem you have correctly identified thus does not arise!
If you’re making 500k, the income bracket increase used to fund the program would cost far more than the gain you received, but realistically it probably would be written to phase out for pure optics.