Since the threat of PR disaster is just an ancillary bit of evidence, rather than core to my logic, even if I accept your point entirely (which I do not), it is not sufficient to destroy my argument. Of the motivations I gave, the one I would expect to be most powerful is the general desire to keep things internal without involving any external resources, which is just a general operational security principle.
I would also disagree that your assessment is correct anyhow; they are insensitive to certain kinds of bad PR, but not generally immune to it, nor do they act generally immune to it. The people in charge of funding the intelligence community may not come after them for getting caught putting backdoors in things; indeed, this may even prove to the people controlling the purse that they are doing their job. But they are certainly sensitive to ensuring that the people controlling the purse do not come off looking bad, and as a part of that, sensitive to not getting caught conducting open, unambiguous acts of war against every nation in the world. We all "know" that they do, everybody else "knows" that they do, and everybody also "knows" that you shouldn't trust Chinese-manufactured electronics either for the same reason, but it's still not openly known. The distinction between open secrets and openly-known facts may greatly confuse Spock and he may shake his head about how illogical it is for everyone to know a thing and for everybody to know everybody else knows but still act as if nobody knows, but is an integral element of understanding human politics, in which appearances matter a lot.