I think there probably is a fairly generous DCI DH provision simply because mother Delta wants it that way because they are so desperate to diversify their "portfolio" of regional providers to every destination. No DCI carrier would agree to pay DH costs to position crews at the whim of mainline marketing. However there is the issue of abuse for all these little Lorenzy Air Group wannabe virtual carriers with a million certificates doing flying for anyone and everyone. Can a RAH pilot DH on a DL flight even though he is going to (or coming from) UAl flying? Is DL really able to track that reliably and do they care enough to do so?