I do understand why they do it, I really do. It's simply human nature to cheat. We cheat ourselves, our families, our friends, usually to gain some advantage or to simply prove ourselves. Sometimes with real consequences, sometimes not. If you know as an athlete that you can achieve great things, know it in your heart, yet fail to deliver on the day(s) in question - be it because of poor judgement, tactics, illness or injury - you are left with a lot of 'what-ifs'. If you are open to temptation, if the EPO or other enhancing substance is available, you have a choice. And choice is what it is all about, and where we become much more black and white, good or evil about these things. Iban Mayo appears to have chosen EPO to gain, or perhaps regain, what he displayed in the Tour. He looked reborn as a rider, best he'd been in years. And now we may know why.
Vinokourov faced the same pressure, but worse; that of a favourite in possibly his last shot at the title. What if he couldn't deliver? What if he injured? What happens then?
And Rasmussen? Well he didn't test positive, he just don't look or sound honest. Or perhaps we don't know the full story? We do know he chose to train away from the spotlight and to appear to prevaricate over his location. This is not illegal, and is our perception only. He wasn't proven to have cheated, and he continues to deny it.