Ok, I did some digging and unless there's a more recent law out there, the one I found states:
Ley No. 285-04, General de Migraci?n. Publicada en Gaceta Oficial No. 10291 del 27 de agosto de 2004.
Art. 117.- Las empresas de transporte internacional, sus agencias o
representantes deber?n:
7. Responsabilizarse por la efectiva salida al exterior, de los pasajeros
que arriben al pa?s en tr?nsito y deban salir del territorio nacional en
forma inmediata, por igual o distinto medio de transporte o desde el
mismo o diferente lugar al de su arribo.
In essence it says that airlines that conduct international travel are responsible for their passengers' exit from the country, whether it's to a different destination than where they originated from or back to the same place where they came from.
If the airline doesn't do it's due dilligence, it can be heavily fined. Airlines may not impose the return ticket, but by not doing so, they're leaving themselves open to potential fines and other punishment from the DR government.
Airlines will waive this requirement for airline (and interline) employees. If you're a regular-fare passenger you will not be legally issued a one-way ticket UNLESS you are a DR resident OR citizen.