It's especially cheap since in The Impossible Astronaut the characters wonder if it might be a duplicate or something, and then Canton shows up and says "That is most certainly the Doctor, and he is most certainly dead".
I mean, you can debate about why he would even know that, but from a narrative perspective it's just a huge middle finger to the viewer. You have characters question if it might be a duplicate, have another character show up to say "it's not a duplicate" with authority, in their only real role in that scene, and then twelve episodes later go "lol, it was a duplicate after all, guys".
I don't think Canton knew. The Doctor uses people, and he was using Canton.
Seriously. The only thing I can figure is that the explanation is so goddamn simple people refuse to believe it after years of convoluted and complicated twist endings.
And the whole season was about duplicates, basically. So it was entirely thematically consistent.