MLS has a complicated set of roster rules that deeply constrain what clubs can do with their money, and what is to their advantage vis a vis the salary cap. It’s no accident: every rule encourages acquisition of a type of player the league considers strategically important.
Homegrown players and Generation Adidas players are off the cap entirely because they help grow the domestic pipeline to the national team. The Designated Player rule encourages teams to spend as much as they can afford on just a few players in order to lure marketable stars. Targeted Allocation Money can only be used on players from the game’s global middle class, the kind that aren’t likely to draw huge crowds but are essential to smoothing the gap between superstars and minimum salary scrubs.