ATX power supplies, as well as other switching power supplies have something like this on their designation label: "Do not remove the cover. Trained service personnel only. No user serviceable components inside" So, basically consumer should not remove the cover and care about what is inside.
Also they are designated to use in wide input voltage\frequency range, not just 110 VAC/60Hz, so it's up to customer where to use them (and different standards could be applied). They typically can work in wide input voltage range, from 100 up to 240 VAC (both 50 and 60 Hz) and sometimes even from 12 VDC, so you can use it in any country and with any input voltage and frequency, just by changing appropriate power supply cable (and plug).
Such supplies are mostly produced on big factories in China and are sold worldwide. So the manufacturer has only one schematics and one production line for all countries with the same wires colors.