Nokia 6 is no longer exclusive to Amazon in the US
Apparently, that exclusivity deal is over now since another retailer in the country, namely B&H, is now selling the Nokia 6. The smartphone is available for the same price you would pay if would choose to get it from Amazon, $229.
Although the US version of Nokia 6 comes unlocked, it will not work on CDMA carriers like Sprint and Verizon, but you won't have issues using it with AT&T, T-Mobil, and any other GSM network operator in the country.
Things that are NOT allowed: