Let me preface this by saying that I don't dismiss the possibility of top players doping (in fact I think they are) and that there are shady dealings going on behind scenes that we know nothing of but this silent ban theory regarding Nadal just doesn't make sense to me.
Tennis has very few genuine stars as it is and while I like both Novak and Murray, they have nowhere near the star power of Nadal (only Fed can be argued to be at the same or probably even higher level in that regard) so given that Nadal is the biggest or 2nd biggest cash cow wouldn't it be more plausible that they cover up his supposed failed doping test then to force him out of the game for an extended period?
Another thing, what makes Nadal so special in terms of doping that he's the only one of the top guys that is caught? Do you really believe that other big names in tennis (Fed, Novak, Murray) don't have access to the same high level stuff he does? Or that they would be able to hang with him if he was using and they weren't?