US (and others) have been doing this for years. It's more than likely, particularly with the recent and near term increase in 330 deliveries, simply using the predefined international route Spares on active domestic routes, rather than just have them sit idle at PHL or CLT until needed. It has little if anything to do with some sudden increase in revenue/yields on the selected routes, but rather an ability to quickly move a spare to PHL, CLT, or in rare cases, an international destination. The Spares may also be planned as replacements for scheduled maintenance.