Yep. Based in DTW and there are numerous domestic markets where DL still has roundtrip fare filings (that are cheaper than two one-way's). For example, the cheapest DTW-DSM one-way fare is $202, while they have roundtrip fare filings for as low as $227 roundtrip.
Combining both market dominance and round-trip necessity: DCA-GSO was $684 round trip and $849 if you booked two one-ways. Such a deal for a 45 minute flight....