In hindsight it is easy to say Lewis could have gone faster, during the race though he had no idea how the tires would hold up and he had no idea how well the tires on the Ferrari would hold up so he wanted to make sure they lasted until the optimum lap for the switch to mediums and then he could extract any remaining life from them.
If Hamilton had gone faster, opened up a 7-8s gap to Vettel but burned out his tires and been forced to do half the race on the Medium it would have compromised them heavily. Especially if Vettel had managed to go another 5-6 laps to have much fresher rubber at the end of the race and that would also have put Rosberg at risk.
Even if Hamilton did it knowing full well what he was doing and he knew he was forcing Rosberg into a position where he would need to defend the undercut from Vettel and thereby not having the ability to attack on fresher tires at the end of the race is that not just playing smart? What is wrong with that, if it was Alonso doing it he would be praised for having superior racing intelligence and would be likened to Prost for his race smarts but if Hamilton does it it is unfair to his team mate because it makes it a bit more awkward for him.