UPDATE: Xiaomi has released a statement on US Mobile carrying its phones as below:
"Recent reports have indicated that Xiaomi products will be available in the US. Xiaomi only offers a small selection of accessories for sale in the US through Mi.com. There are no plans to sell smartphones through any authorised distributors in the US. US Mobile is not authorised to sell Xiaomi products in the US."
This explains why the models on sale are not compatible with US 4G standards and it's likely US Mobile is getting the phones via alternate channels.
Xiaomi phones are now on sale in the US thanks to carrier US Mobile. It's not alone, though. Competing Chinese brand Meizu also has one of its models, the Meizu Note 2 listed.
Xiaomi has an online US store but until now, it had only been selling accessories such as headphones. The models listed by US Mobile are older ones, namely the Xiaomi Redmi 2, Mi 3, and the Mi 4. Pricing is US$119, US$135 and US$219 respectively.
The phones on sale aren't compatible with US 4G standards but the cheap prices might appeal to consumers wanting decently-specced Android phones that don't cost too much.