Assume that the airline is amongst the Signatory Visa Waiver Program carriers (mirror). The Visa Waiver Program Carrier Agreement (CBP Form I-775) that such an airline must sign to be part of the Signatory Visa Waiver Program mentions:
The carrier will issue only tickets that are roundtrip, nontransferable, nonrefundable (except in the country of issuance or nationality or residence of the alien), that are valid for one year for purposes of deportation and exclusion and guaranteeing transportation from the United States to aliens seeking transportation to and admission into the United States under the Visa Waiver Program;
Does it mean that airlines are only allowed to sell roundtrip tickets to individuals who want travel to the United States under the Visa Waiver Program? (i.e., they are not allowed to sell one-way tickets.)