Earlier quoted context omitted.
Well, if you're Apple/Amazon you have the SEC and your shareholders ready to ruin your life if you say anything that could even be interpreted as a lie. So, in this hypothetical you may not want to, but you will definitely feel forced to.
You can also have the government out to ruin your life if you don't cooperate. See: Joseph Nacchio and Qwest
I could believe it if the denials so far felt incomplete or ambiguously worded as if they were tiptoeing around something that they were not allowed to disclose. I could believe it if all we had coming from Apple and Amazon was the usual lawyer-speak "I won't confirm of deny" bullshit. Instead we've had completely unambiguous "this is completely false and never happened". If it turns out to be a lie it's going to be devastating for the trust in Apple or Amazon.
I mean think about it, if for some reason the US or Chinese agencies wanted to downplay or shift the blame they had so many easier ways to do it that would put them in an awkward position if somebody manages to prove the existence of these backdoored mobos. If the best spin they could come up with was "just deny everything and make sure to do so at a regular interval so people are constantly aware of our denial" they really need better PR people.