To your question: Relative transparency and relative accountability of the governments in question.

But your question is by nature setting up a false comparison. There is no observed censorship apparatus in place in the US. You can always pose "what if" scenarios about super classified efforts, but functionally there is no active system in place.

Compare this to the vastly-resourced, all-encompassing one operated openly and enthusiastically by the CCP.

Short of absolute transparency and absolute accountability, you can only be relatively certain, i.e. not certain.

But not all “not certain” are created equal. After all, if you apply a sufficiently strong threshold nothing is certain.

On the basis that there is no automatic censorship of user content shown to me via iMessage. I am highly certain that my friends can send me arbitrary (within technical reason) text and images and not get censored. This is not the case with WeChat in China right now.

Given societal norms I think there is a strong expectation that my iMessages will not be censored in the near future either.

This means there is little incentive for NSA to train and develop an actionable automatic censorship application.

Again, this is true even if they intercept, decode, analyze, commit physical action on the basis of the analysis, and store every one of my messages.

Ah, sorry, I erroneously read "censorship" as "surveillance".

I suppose there is not really the same kind of overt deployed censorship in the US like there is in China. I do personally think that the NSA somehow gets most messages sent in the US, but I have no idea what they would do with it or if that hypothesis is reasonable.