I don't believe every time a player contracts rhabdo, it's because of illegal PED use. But 13 players didn't all contract rhabdo after a workout simply because the workout was too intense. There's another factor in there that Iowa fans don't want to hear about.
Actually the fact that 13 contracted rhabdo after the one workout does indeed point that the workout WAS the cause. There is more evidence that creatine is a contributing factor to rhabdo than anabolic steroids.