These stories are chilling! I did a turnaround once in Frankfort--was in the airport for literally less than two hours and flew back on the same NWA plane I came in on. Braun Hilda at check-in was very curt in her questioning, but no hassles.
I prefer to do runs within the U.S.--free upgrades to FC for all domestic flights (except Hawaii) on NWA, so I can sit up front. Last MR was from DTW to SFO and I got into SFO at 11:15 am and flew out at 12:35 pm--same plane, same crew (my ticket did not require a Sat night stay--watch carefully, they do pop up!). Sat in F both ways and nobody from the crew ever said a thing, and no hassles with customs and/or immigration. True, I can rack up only about 5000 miles RT on one run, where you can tally much more on an International flight, but the upgrades into F are worht the multiple runs--and they can be done in one (long) day.