He only admitted it after there were front-page stories about it, leads on radio news for days, admissions from law-enforcement that they were investigating the dealers... yep, it takes a Big Man to admit something at that point... after everyone knows the story!
Be honest -- if Rush had admitted this BEFORE everyone knew he was in hot water, your point would be valid. Not now. He's only admitting it because he knows he's been caught, and going into rehab might mitigate possible leagal repercussions because of it.
He didn't admit anything before or after his first TWO attempts at rehab.
Why would he have to tell anyone?
He's an American citizen, not an elected official doing the taxpayers business.
Do you tell the world everything you've ever done? Why not?
Because it was nobodys business but his own and perhaps his wife's. Do you tell everybody everything about you? Hmmmmm? At least when the word got out he didn't resort to the "I didn't have sex with that woman, Ms. Lewinsky, defense did he? You can't see class even when it hits you in the face.