The classic nested ticketing they are trying to prevent is the guy in ATL working in NYC most every week who flies on interleaved tickets to avoid the higher priced Mon-Fri lights.

If what you are seeing is cheaper fares that just happen to be interleaved, that is not in violation of DL's written rules, and nothing to be worried about.

EDIT: I am assuming that the flights not violate a stay restriction of some sort if booked "normal".

EDIT2: No idea about the GUC issue.