It's to ensure competitive balance. That'd be like giving candy to a kid who's already won the prize.
You're "bonus" is by virtue of the fans coming into your arena to watch the champion team ball out on the court. You get more season ticket holders and can (if you do it smartly of course) set higher prices than your average League 1 level team, at least for about the first half of the subsequent regular season.
Bonuses are given to lower league teams advancing to a higher dvision as a way to help them get acclimated to the tougher terrain of said higher league. Whether they use it to buy more players, upgrade their arena, or infrastructure, its up to them how they handle their money. But the penultimate goal the game sought for was to help those newly promoted teams out a bit to help level the playing field due to those incumbent league teams A) being more established and
were able to foster more lucrative salaries and payroll, thus also having ability in higher division to rake in more Sales revenue via Arena sales and Merchandising.
3 Time NBBA Champion. Certified Trainer. Mentor. Have any questions? Feel free to shoot me a BB-Mail!