Actually, if how Facebook is installed on Chinese iPhones holds the same, they just need to switch their phones over to the Chinese App Store by setting their region, install WeChat, and then set their region back to the USA. Apps from the Chinese and American app stores can easily co-exist, and worked well for me while I was living in China.
Apple would presumably be required to prevent such workarounds to the best of their ability if the ban came to pass. (I'm honestly surprised to hear that China hasn't forced Apple on the issue.)
That China hasn’t been able to force the issue might mean the USA isn’t either. Geographical blocks are super hard to implement without a firewall, I’m betting that the USA will just be happy if it is off the American App Store and not the other ones.