How do you define "cheating"?

In order to test for something you have to define what an acceptable level is. It's obvious to me that the TDF teams are testing their riders blood levels daily and boosting their blood levels just enough to keep them at, but not to exceed, the testing threshold. That's why they can pass the drug tests even though witnesses report observing drug use.

If Contador's blood was right at the threshold level beforehand, I suppose it's possible the beef he ate really might have given some hormone level just the boost that was needed to push him over the top.