I was wondering about this, too. Considering that NSAC can apparently opt to not disclose the results of drug tests indefinitely (1 month+ between the positive test and Jones' announcement), I wouldn't doubt that Nike's contract with Jones contained a stipulation that test results would have to be disclosed immediately and could be considered grounds for termination.
Nike executives are no chumps. They deal with young, multi-millionaire, party animals on the reg. I'm betting they've all sorts of contractual hedges to protect against shit-storms like this. To me, this now looks like the most likely reason Nike dropped Jones. And since Nike probably would be reluctant to come out and say, "Yep, we sponsored a cocaine addict," they never contradicted Jones' "it was a mutual breakup" nonsense.