It explains how Australia could coerce Apple into performing widespread surveillance of Aus citizens. Then it’s trivial for the US to coerce Apple into switching that functionality on in the US.
Any widespread warrantless surveillance of the private physical property of US citizens, performed at the direction of the US Government, would be an absolute clear-cut unambiguous breach of the 4th Amendment.
I'm not saying the US Government wouldn't care that it's unconstitutional—we know they'd ignore the constitution when they can get away with it. But they'd also have to convince Apple's lawyers to go along with unconstitutional surveillance. You don't think Apple wouldn't be itching for another opportunity to prove their strength against a Government? Especially now? Apple would love nothing more than to have more opportunities like they got with the San Bernardino iPhone.
I'm not saying the US Government wouldn't care that it's unconstitutional—we know they'd ignore the constitution when they can get away with it. But they'd also have to convince Apple's lawyers to go along with unconstitutional surveillance. You don't think Apple wouldn't be itching for another opportunity to prove their strength against a Government? Especially now? Apple would love nothing more than to have more opportunities like they got with the San Bernardino iPhone.