By Julien Pretot CARCASSONNE France (Reuters) - On the first rest day of the Tour de France, Team Sky boss Dave Brailsford wondered why Vincenzo Nibali was being let off the hook over cycling's doping problem after last year's champion Chris Froome was grilled multiple times on the matter. Froome answered dozens of doping-related questions as media urged him to prove his credibility in 2013 but until Sunday, the day before the second rest day, Nibali had faced no more than three questions on the subject despite comfortably leading the general classification. So why is Nibali having an easy ride on a theme that has dominated the sport for years? After all, Nibali is riding for the Astana team, whose manager Alexandre Vinokourov was banned for two years for blood doping in the 2007 Tour de France. More...