In a lot of cases airlines will block people from flying to a foreign country if they have a one-way ticket because the airline expects the destination country

In a lot of cases airlines will block people from flying to a foreign country if they have a one-way ticket because the airline expects the destination country will block the person from entering the country because the one way ticket implies they are not visiting as a tourist. In that case the airline is required to fly the person back. This is more common now with various Covid-19 travel restrictions so airlines are also asking for all testing paperwork that is required for the destination countries.