Flew LHR-SFO-PHX-ORD-LHR about a month ago. All UA and all except the ORD-LHR had one thing in common: neither the CC nor the PAX took one little bit of notice of the seatbelt signs. (Sorry to pick on one airline, I'm not saying that they're worse than anyone else...)
On the LHR-SFO leg, the CC were standing in a huddle next to my seat complaining to each other that the FSB sign was on and the PAX weren't sitting down. Not one made any effort to go and speak to those concerned.
The only person who made a difference was a steward on my last flight who, as soon as the signs went on, sat down on the crew seats beside the toilets (middle of cattle class, 777) and forcibly told the wayward PAX to go back - and cross their legs.
On the UA flights in question, they always repeated the same phrase at the end of the safety demo. "Federal Law requires all passengers to comply with lighted signs, placards and crew member instructions" (see, I was awake for those 3 minutes
). So why not invoke Federal Law? On arrival at the gate, the sight of a big Marine + semi-automatic coming for Mr Awkward in 12C would probably make the others think twice on the return journey!