> Because NSA can't legally "spy" on US citizens, they would be happy to allow UK, or say NZ, to access those backdoors and record everything US citizens do.
This is, modulo using the hypothetical backdoors under general discussion, pretty much exactly what the "Five Eyes" member states (which set includes .nz and .uk) are doing for one another already.
Because NSA can't legally "spy" on US citizens, they would be happy to allow UK, or say NZ, to access those backdoors and record everything US citizens do. Then they just have to find a loophole in how to access that information later. Granted they can probably just redefine what "searching" and "access" means, which do already, but this would open even more loopholes and possibilities.
It's sometimes useful to think of these government agencies not as working for the US citizens but as adversaries who work against our interests.