Because despite what the brain says, you can only get from your body what is in it. The usual technique is visualisation, seeing yourself performing well. It's also useful in counterbalancing mental pressure of expectation. One thing to push through the pain, helped by positive messages, entirely another to get "help" from physical "preparation".
I'm not convinced about the Lemond arguments, his allegations. After all, attack is the best method of defence. And anyway, why now and not just after his career? A cynic might say because there are no old stored samples that could be retrospectively tested?