If you find that someone broke into your house and stole your Xbox, and there's a guy down the street who you already know has a long criminal record of home burglaries, is it unreasonable to suspect that guy to be a likely culprit?
Likewise, if there's a backdoor in every new PC's CPU, and there's a three-letter agency presiding over that CPU vendor's jurisdiction that has a long track record of backdooring things, is it unreasonable to suspect that maybe - just maybe - that TLA is responsible?
Now of course the ex-con or the TLA can be totally innocent here, but if I were a detective, they'd be the first ones from whom I'd be asking for alibis.
The hypotheticals aren't comparable, nor is the prior available information - and even though the Intel AMT/ME situation is egregious, explaining it simply as "a backdoor" is an oversimplification.
I'd agree that probabilities with regard to what is reasonable to assume (the operative word here) shift as a consequence of circumstantial information, but you can't really draw any conclusions based on that - hence the burden of proof, i.e. the presumption of innocence in your analogy.
Likewise, if there's a backdoor in every new PC's CPU, and there's a three-letter agency presiding over that CPU vendor's jurisdiction that has a long track record of backdooring things, is it unreasonable to suspect that maybe - just maybe - that TLA is responsible?
Now of course the ex-con or the TLA can be totally innocent here, but if I were a detective, they'd be the first ones from whom I'd be asking for alibis.