The reason that salaries have to match *FOR TEAMS OVER THE CAP* is that, being over cap, they're not allowed to take on extra salary. Keep in mind that if a team is under the cap, they can take on whatever ridiculous contracts they want (up to the cap). If Miami wanted, they could trade Bustley for T-mac (let's pretend they have that much cap room; I can't be assed to check right now).
So the question becomes - why should a team over the cap be allowed to take on more salary (other than the exceptions, which is another issue entirely) when they're already at their ceiling? Answer: they shouldn't.