I'm torn when i try to answer this question. My first thought is that he should surely be denied TdF entry. Cocaine is illegal, unhealthful, dangerous when operating motorized vehicles, and very addictive which can lead to a whole collection of undesirable outcomes.
But my second thought is that he tested positive out of competition for a "recreational" drug, which then categorizes it as part of his personal life. If he tested positive for it during competition or just before competition, close enough to the event where it would still solidly be in his body while competing, then that would be different.
Another thought to consider is that when he chose to be a professional cyclist, he took on the responsibility to be a role model. Role models should not take illegal drugs.
Here in belgium it seems that for the most part Belgians are in support of Boonen. It does not mean that they condone his behavior, though. I think that people here are understanding that athletes are just like everyone else.
I am curious to see what people think. And feel free to vote in my poll to the left.