Tamba Hali was suspended for one game under the NFL's substance-abuse policy because of a previous violation of the law, not because he tested positive for illegal or performance enhancing drugs.
Reportedly, Hali and the Kansas City Chiefs were able to hide the news that Hali had been arrested and prosecuted for a crime and this was the basis for the suspension by the NFL. The prior violation of law was unreported and undisclosed and therefore went unpunished by the league, which is now correcting that oversight.
According to Pro Football Talk, the suspension under the substance-abuse policy is due to the original arrest being related to illegal drugs.
It's our understanding, per a source with knowledge of the situation, that Hali's offense likely involved marijuana, and that (as the league has announced) the penalty was a one-game suspension and a one-game fine.
The good news for the Chiefs is that it doesn't mean Hali is in one of the advanced stages of the substance-abuse program, which would have put him on track for, eventually, a one-year banishment.
For more information on Hali's suspension, check out Arrowhead Pride.