No team can pay more than $X per year in total salary!
Players are not allowed to move freely between teams!
They can do this without violating anti-trust law because NBA players have a union (the National Basketball Players' Association, NBPA). The NBPA negotiates with the NBA on behalf of its members, and agrees to these rules. The union also does things like make sure that 50% of league revenue goes to players.👍🏿