The New York Times ran an article Saturday talking about how Troy Glaus and a couple other Angels took steroids to get healthy and keep their jobs in the early part of the decade.
For all the ARod hysteria, a lot of these guys just wanted to get healthy and not lose the massive paycheck. If you put yourself in the shoes of a player with a family to provide for, the decision is almost a no-brainer, so long as you believe that the roids are the only way to ensure your continued production. Do you really care that much about whether it's against the letter of the law when half the guys around you are doing it, and your livelihood is at stake?
Now ARod doesn't fall into that category - but even if you don't need the roids to be great, you're still competing with the Troy Glaus-types for home run titles, etc.. and if they're benefitting, it makes sense to want to try it, too - again in a culture where half the league is doing it.
As more and more information trickles out - as it surely will over time - it's going to reveal that the culture and system encouraged the use of these substances, and while the individual players ideally should have said no and rehabbed injuries or worked to get better without this major boost, it's management, ownership and the commissioner's office which was responsible for the systemic problem. You can't expect the players to police themselves when they have so much to gain from using the drugs and in some cases a lot to lose by not using it. Baseball itself had nothing to gain from Glaus or ARod being on roids and a lot to lose, and the commissioner's office is in charge of looking out for baseball.