> But the irony is that the device manufacturer could easily disable this backdoor in favor of their own.

I'm not sure it works that way. Perhaps the NSA could force certain US companies to specifically design and install backdoors, and require these backdoors to be always active. (One of my possibly ungrounded/paranoid fears is that they do this with Intel ME).

The balance of likelihood is they do on both Intel and AMD management engines as that's what any competent agency should do in this age. To my mind, there's no other real reason for the explosion of interest in ARM chips on server. ARM design is sufficiently well understood and simple that usable chips can be designed and fabbed away from the control of US companies even if it provides poor performance trade offs.

When I was a little boy, I heard a myth about argument between the US and Japan, The US said, we can remotely bomb any place that has a Television, and Japan replied, we can remotely bomb any place that has a Transistor Radio. I totally believed that back then.