"Why would Floyd do it?"

That's a question that the media (probably, with plenty of prompting from the Landis camp) are floating around to try and maintain the illusion of his innocence. I have a simple explanation for why he would take an easy-to-detect synthetic compound and jeopardize his name, his future and his legacy: simply, he didn't expect to win.

Which would mean that the lower down in the Tour he finished, so would his chances of being tested. From a numbers standpoint, that probably justified the risk in his eyes, especially with his future in cycling questionable because of his upcoming hip replacement. He probably figured he'd do well enough to maintain his spot in the US team and maybe earn more endorsement money, but not well enough that he'd draw attention to himself and find himself in the national spotlight like he is now. Remember he wasn't favored to win.

I have no question in my mind that he cheated knowingly.