oh Ive read some of the media on this. You guys have most of it wrong. Rios came out on Chilean radio saying that the ATP plays favorites when it comes to its most popular players (NOT that Agassi took drugs) and that when it comes to players like Agassi and Sampras, etc, guys who have buttressed the sport, the ATP never publicly discloses their drug tests while opting to publicly disclose those of the "lesser" players. He says that if Agassi had tested positive for nandrolene (a drug thats more common than you think) the ATP would never think to disclose it for economic reasons. Such a scandal would be too big for them to handle.
Its important to point out that Rios here never accuses Agassi or Sampras of taking drugs, but rather that the ATP goes to large efforts in protecting their reputations over others. He says no one has any way of knowing if whether or not the atp actually tests agassi or gives out the right results --there's no secondary independent body to certify such results along with the results published by the atp.
In other words, he and "the other south americans" [his words] have long speculated that the atp plays favorites when it comes to drug testing and accuses the atp of just being a little too sketchy in its public disclosures. He says the process should be more transparent.
Hope my explanation helps.