The reason other sports haven't had any problems is because the clubs have agreed that they will not challenge it
just like in other sports they have a draft but in RL due to Terry Hill it's been ruled illegal if anyone from the NBA, NFL, NHL or AFL challenged their sport's draft the same result will happen.
Just like if anyone from NRL, AFL, NHL, NBA, NFL etc challenged their sports salary cap it would be proven illegal.
While I don't agree with anyone challenging the cap because I think it does more good then harm, regardless of if it's Politis, the players or another club the fact remains that it is a restraint of trade and is therefore illegal and it's only a "Gentleman's agreement" between all parties that the cap hasn't been challenged in the courts.