Why Would He Do It?
Conveying American History X's complex theme (that racism is wrong) required a chiseled and menacing figure to represent white supremacy. And, let's face it, if white supremacists all looked like the "before" picture up there, it'd be hard to take racism seriously as a problem. Society could simply put racism in a headlock until it made a tearful apology.
Sylvester Stallone
Before & After:
Sylvester Stallone has really been in shape his whole career. Here, he is just casually hanging around his house.

Here, he is once he finally got approval for Rocky Balboa ...

... and here he is training with Tony "The Grizzly Bear" Mahoney between takes.

So, what's our point? Well, remember Rocky III when an aging Rocky had to fight the young upstart Clubber Lang? Well, here's what Clubber Lang looks like today.

Sure, Stallone has always been in shape, but he's 60 fucking years old. It doesn't help his case that, while promoting this film, Stallone was stopped at an Australian airport and charged with illegally importing a steroid known as HGH (human growth hormone).
He Claims:
"To [customs officials] it's major, but it's really minor stuff. I just made a mistake ... I misunderstood a few things, and we are going through the process."
Other celebrities can definitely learn how to defuse their own drug situations by studying Stallone's clever use of words. Sly's use of steroids is just a "minor misunderstanding," Britney Spears is guilty of "confused drinking," and Andy Dick has a "silly cocaine hobby."
So Is It Steroids?
Yes and no, though mostly, (if not totally), yes. Stallone did plead guilty to possession of testosterone and human growth hormones, but he also said it was a "mistake" made because he was "ignorant about local laws." Ignorance, as authorities have repeatedly pointed out to the CRACKED staff, is no excuse, particularly when "Don't do steroids" isn't exactly a "local" law.
Why Would He Do It?
Do you remember all the subtle, character nuances that make Stallone's movies so iconic and inspirational? Of course you don't, because there aren't any.
Now, do you remember when Rocky punched all those guys and, in later movies, those other guys? And, do you remember when Rambo made someone explode with a single arrow and laughed off fatal bullet wounds? Fuck yeah, you do! It's pretty simple; Stallone gets work because he's an enormous ass-kicking machine with indecipherable speech patterns. If Sly ever hoped to get another job, especially at his age, steroids were probably the only course of action.