Applicable to only those of the US (who choose to comply), which would mean giving non-US an advantage, which is a distinct no-no. They tried to restrict Fable to only-US, but that backfired since the world isn't at all clean cut, resulting in full unavailability of the model
Without international treaty and regulation restricting frontier capabilities globally, any attempt to outlaw open source models will only be as effective as King Canute ordering the tide to turn. Unless the USA fancies bombing those who refuse.
It cannot. The only available law is an export control law. A restriction on an open model would be strongly unconstitutional due to a freedom of speech guarantee in the Constitution.