NTSC just needed a different modulator; and the ZX81 used a simple transformer/bridge rectifier/smoothing capacitor brick to convert to 9v DC, with a regulator

NTSC just needed a different modulator; and the ZX81 used a simple transformer/bridge rectifier/smoothing capacitor brick to convert to 9v DC, with a regulator in the computer to convert down to 5v, so 60Hz wouldn't have been an issue, although the mains power connector is obviously different. So, there's nothing special about a machine for the US market that required it to be manufactured there. Presumably it was commercially preferable to do so. You're correct though that a US model wouldn't work in the UK without modification, and vice versa.