Thing is Froome was crap 2 years ago, and suddenly became one of the best riders in the world, coming 2nd in the Vuelta in 2011. His "pedigree" comes from that point onwards. They say it's because he was ill before, and this is what he should have been doing.
Call me a cynic, I want him to be clean, along with Wiggo and the rest of team Sky, but I have my doubts. Maybe it's because of the similarities between Sky and US Postal. Both leaders and teams have dominated, both have the same answers to questions on doping, in the same way, both say the reason they are better than everyone else is from marginal gains etc.