US networks get cagey about certain phones. CDMA carriers were always restricive, but now that 2g and 3g are mostly shut down, they don't want to allow registration of LTE devices that don't do Voice over LTE with their network because it breaks the emergency service rules.

I don't think they'd block them from roaming if the device has a foreign network sim that's authorized to roam though. Of course, major US carriers basically don't allow their subscribers to roam to other major carriers.