If you played Battlefront II, this is what they had to say about it (the Retired 501st Trooper narrarating...):
"The bounty hunter left after the battle was over, said something about tracking down a smuggler on Tatooine. After the Kamino uprising, the Emperor decided an army of genetically identical clones was too susceptible to corruption. Future clones would be bred from a variety of templates. Though the 501st itself remained pure, gradually, the rest of the Imperial Army became more and more diverse. We never really got used to the new guys."
So, the 501st was pure, but not the rest of the army...
