ok so they fly to another country that flies into the usa. us citizens arent allowed to fly to cuba but many do thru other countries.
semp
When you increase a barrier to entry, you get fewer people entering. For example, eliminating flights from Liberia would result in a significant reduction in number of Liberians travelling to the US.
However, if we were going to do that, it is trivially simple to ban travel into the US for people with Liberian passports regardless of where they originate.
Of course, currently, the simplest way around any restriction is to fly to Mexico and walk in with the rest of the 20,000 people per month doing that.