Why are riders allowed to draft behind cars in Grand Tours?
It is not allowed by UCI rules, but comissars usually allow it if it is due to mechanical reasons and used to get back to the peloton, since they have discretional ability to decide. Time penalties or disqualification if used to gain advantage over the peloton. So, rule enforcement may vary depending on many circumstances, and I guess they don't want to lose stars like Froome, that could also be important when turning a blind eye on some minor issue