Good Question!
It's because if they were not tied together you could get a situation like this:
You adjust your minimum brake to 5%
You have your drag brake set to 20%
When you let off the throttle your ESC will apply 20% of maximum braking force.
When you move to push brake, the ESC would appl 5% of maximum braking force... LESS brake than at neutral...so your car would actually have less brake than at neutral...it would sort of "run away" from you.