They are able to fly to the US and walk around cities such as Houston, Boston etc with no restrictions.
Not true! There are a lot of restrictions for foreign carriers.
COVID-19: Interim Health Guidance for Air Carriers and Crews.
amongst others:
o Avoid contact with sick people
o Stay in their hotel rooms to the extent possible
o Minimize going out into the general population
o Use social distancing (maintain a distance of approximately 6 feet, if possible) whenever out in public
o Avoid crowds, stores, sporting or mass entertainment events, and other situations likely to attract large numbers of people
o Eat within their hotel rooms with either room service or delivery service. If in-room dining options aren’t available, they should eat at a restaurant located in the hotel. If not available at the hotel, they should eat at a restaurant located close to the hotel.