They've had separate one-way vs. roundtrip award pricing on routes (including domestic) pretty much ever since they got rid of charts and went to dynamic pricing around a decade ago (see below). They just largely track revenue fare prices and differentials on one-way vs. roundtrips revenue fares have likely been increasing recently. Routes without much competition (particularly LCCs like WN) are more like to have roundtrip fares than those with substantial competition. The legacies largely match WN fares on routes with more than one or two daily non-stop WN flights and WN only has one-way fares. B6 also only has one-way fares.

Significant Increases in Skymiles Redemptions

Domestically, roundtrip fares and awards require staying at least until first Saturday after departure (or at least one night if you depart on a Saturday). If you don't meet this requirement, you just end up booking two one-way fares/awards with a roundtrip booking and pricing will be the same as two one-ways. The wording in award fare rules is a word-for-word copy of revenue roundtrip fare rules. Unfortunately, award fare basis code/fare rule display on delta.com has been broken for several months now. Awards with a roundtrip booking requirement will have an 'R' in the middle of their fare basis codes (you can find fare basis codes in "Fair Details:" line in email receipts).