The reason for it is that the league, in itself, is a source of free marketing.
For example, right now, if I wanted to start a private investment fund, I'd be limited to a few friends/family who'd want to see many years of a profit/loss statement before they even commit to giving me $10K.
On the other hand, if I were in Rondo's shoes, I'd first of all, have my own first million to trade with. And in a short time, I could make some $400K-$500K, with it, spread the word to reporters, etc, and a lot of well heeled individuals would want to invest in Rondo Capital Partners. Then, when my max contract is up, I'll already be making some $10 million a year, w/o a Nike deal, and I could re-sign with the C's at veteran's minimum and bring more and more rings as I play alongside Durant, Stoudamire (off the bench), and others.