Basically, until we have a concrete proof (such as Armstrong in cycling), it is extremely cynical to assume that the same is true in tennis at the very top of the game.
You do realize there is no "conrete proof" about Armstrong being a cheat, right? He was never tested positive... and yet, even before this whole case, it was not "extremely cynical" to assume he was doping. Pretty much everyone following cycling knew it.
For all of those who think the sport is perfectly clean: what do we have to lose with more testing? The only thing I can think of is "money", but surely there is money enough for that. Cycling is a much smaller sport and they are testing about 100 times more out of competition.
Another question for you, who think this is all "conspiracy theory": why would a tennis player NOT dope? There are benefits to be gained, and the chance of getting caught if doing it right is literally zero. The only reason I can think of for not doping is to think every single top tennis player is a gentleman who believes in sportsmanship and fairness.