Apple is in many countries, with governments who'd all like this kind of access. If you give FBI the tools, you also have to accept Russia, China and the whole gang having access to your phone.
And then build that fence all along the border so that no world-version phones get in (see how well it worked for other illegal imports). Plus, you haven't solved the problem at all: you just say "okay, let's make only US phones vulnerable to foreign hacking." Should I enumerate why that's a bad thing?