It isn't about blocking attempts to overtake though, it's about the number of blue flags ignored. Sutil had blue flags waved at him from around turn 3 onwards, that I saw on the TV. It wasn't until the straight that he "let" Hamilton past. That's not far off an entire lap. Sutil is even quoted as saying he let them past after "just" a lap. But that's not how the rules are written and supposed to be applied.
The problem that the blue flags tries to counter is that of turbulence. As soon as Hamilton closed with a second or so of Sutil he couldn't progress any further due to the loss of downforce at the front of the car. The blue flags are to stop lapping cars from being held up too long in this situation, as it's often very hard for the leaders to pass slower cars in a timely fashion doubly so when it's an out of position fast car like Sutil.
Sutil failed to give way within the requisite 3 blue flags and received a penalty. Utterly fair in my opinion.