He doesn't take steroids.
He has been amazing people since he was a young teenager...contrary to popular belief, he didn't 'come from nowhere.'
I find it very sad that you can observe a feat so amazing and dismiss it, because of your preconceived ideas. 
you got it bang on.
humans need several things in order to commit the act of cheating...
1. motive,
2. opportunity,
3. benefit.
Usain doesnt have 1 or 3, as he already was kicking ass for years prior..unless i guess he has been using since he was 13.
he's probably had opportunity.
he's got more to lose than gain.
besides, we've got retrospective testing where they store your bloods for decades on ice, he gets tested 100 times per year, and - if he was using, wouldnt he do it more subtley?
finally, he exhibits no physical indications of drug use...and none of the traits those busted before him who were so so runners who suddenly improved to world beaters, or amazingly improved in their mid twenties..
usain isnt 23 yet, ran his first 100m only 16 months ago- which was 10.03, and didnt even train seriously until 3 years ago...
the guy's a freak..