Why are riders allowed to draft behind cars in Grand Tours? During one of this week's stages of the Giro d'Italia, Richie Porte had a 'technical' issue with his bike that caused him to lose a number of minutes from the peloton. It was of significance as it was less than 10k from the finish and as a result was always going to be difficult for him to regain the time lost on the other GC contenders.

His team mates obviously did their best to pull him pack to the rest of the group. This included the whole team drafting behind one of the Team Sky cars for a period of time.

My question is: Why is this allowed? 
Surely he/they are gaining an unfair advantage over the rest of the riders. 

Addtionally:


Would it have been the same if he'd have fallen off 10k from the start line?
Would he have been allowed to do so if he was two minutes in front of the peloton when he crashed?
How is it decided what drafting is appropriate, and what penalties are handed out when it is deemed the riders gained some advantage they were otherwise unentitled to.
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