Chewbacca
They are/were (don't know the state of chaos at the moment) turning away those with entry visitor visas which they had to go to the US embassy in their country to get. Business travellers, tourists, etc. So they want to put in more extreme vetting procedures at these US embassies to get the visa. Not very good for those who already were on the plane and the airlines who had to send them back at their expense. The airline industry was not happy about non-consultation.
Some students, even with existing student visas who had been in the US at school already, returning from holidays with family were stopped from entering, too.
Those who have applied for settlement visas DID all go through the several months of processing already. My husband emigrated back with me in 1993 and even then it took 3 months for him as a Brit.
The ban caught out those who had their settlement (provisional green cards) visas in hand from those countries... and you have a face-to-face interview when you arrive for the first time as well and must designate which airport you are arriving to. The vetting was very strong. I cannot imagine it has gotten easier. So why ban them?
Existing settlement visa holders, like Mo Farrah, who were out of the US and returning originally had some problems, but that got sorted.