Your logic makes sense, yet these instances grow bolder as well. Torture in Irak. The work of a few evil grunts on the loose.
Google collecting Wifi payload in streets all accross Europe? A programming mistake.
Countless examples where one low level rotten apple is to be held responsible whenever caught red handed.
This communication strategy works so well that it is becoming the standard answer whenever a totally wrong political or corporate policy is being exposed for what it truly is.

So the question remains, who is going to reverse engineer the millions of lines of codes to discover the backdoor?
Probably nobody.
But suppose a backdoor is found, it will be the work of a single individual coder with low moral standards and he / she might get fired. That is certain.