Because, in terms of the politics, that's the red meat (pun only slightly unintended.) Their being 'communist', and communism being percieved as atheistic, anti-capitalist and militaristic, is what makes them the bogeyman, that's why the media and the government go out of their way to make their efforts seem so dangerous and terrifying because being 'communist' makes them our de facto enemies (because we're the God Fearing Capitalist Shining City on a Hill.)

If China was a predominantly Muslim country, plug 'terrorists' into the same socket, it would amount to the same difference, in terms of the rhetoric. If we found out Britain was spying on us (ECHELON aside), or Germany or Poland, whomever, it wouldn't have nearly the same sort of ominous sense of 'the other' that the spectre of China/Russia/the Middle East do.

I certainly didn't mean to imply anything tacit about China being communist having anything to do with their attempts to expand their influence or anything. It's what they should be doing.

One could also present them correctly as a very authoritarian state, antithetical to personal freedom and civil liberties. Maybe there's no political benefit to be gained by saying that.

True.