I can think of 5 possible answers and I am interested in which this community thinks is the correct one.
1) It has always been cheating since the very first time a WWII vet returned to MLB and popped one to restore his focus and improve his energy.
2) In 1971 when the Controlled Substances Act outlawed their use in the US without a valid prescription.
3) In 1991 when Fay Vincent's Drug Policy Letter declared that the use of any illegal drug was strickly forbidden by any MLB or MiLB personnel including players.
4) In 2006 when the MLB and the MLBPA agreed to randomly test and punish for their use.
5) It has never been cheating as even today it is easy to get a "therapeutic use exemption" to use them.