Re: "Think of the children" or something
I find it very hard to understand the reasoning of the US government on these things, and with the current administration, I tend towards "playing silly buggers" conclusions. (nb: if you're reading this in the US, you may disagree, but it's probably fairly representative of how the rest of the world views the current US government)
The argument against protectionism here is that the US doesn't have a significant capability to build (mobile) networks in-house. Cisco has capabilities, but nothing like the big scandinavian players and the chinese. The remaining argument against this being genuinely about punishing ZTE and securing the US against chinese state influence on critical infrastructure is that it's just part of a trade war game, either bargaining with china or appearing tough for domestic purposes.
All told, I think this could be explained in many ways, and we may never know why...