While its pretty dumb what the guy did I also think the dress code is pretty outdated and dumb as well. If the whole idea is NRSAs should blend in and not be noticed then the current situation is perfect for 1975 but no so much today. I have only once ever worn dress shoes on an international flight and that was only due to the fact I ended up out way way too late in Munich and had no time to change before running to the airport and even then the shoes were on only until takeoff and them my sleeper suit and slippers were on in a flash. While I understand they should have some dress code it could use an update IMO and should be more inline with what most pax wear, and to that end tennis shoes aren't exactly outside the norm.