If he did somehow manage to outsmart the test and pass while still taking illegal drugs, then there's something wrong with the test. I don't condone cheating, but let's actually prove he did cheat before we crucify the guy.
The thing is, for most of Armstrong's "reign", there was no way
of testing for the things endurance athletes most commonly used at the time (EPO at first, then after EPO tests were finally developed more than a decade after it became common (!) autologous blood transfusions enhanced with micro-dosing EPO and testosterone).
So why shouldn't normal police investigative methods be used to reveal doping cheaters as well? Certainly it's nice in a murder trial to have the suspect's finger prints on the knife, but it's not necessary
if he committed the stabbing in front of dozens of people.