The rules for duty time are complicated and AFAIK can be somewhat different for pilots versus FAs. Also AFAIK some of the rules are mandated by the FAA for safety reasons but others are bargained contract provisions.

Once I was on a TPAC flight (not UA) where the FAs timed out but had the choice of doing the flight anyway or staying in Tokyo. Most decided to stay there, but a couple were eager to get home for personal reasons and decided to work the flight anyway (one had a friend on the flight as a passenger). Those who didn't stay on were replaced pretty quickly by another bunch of FAs who had been scheduled to fly a shorter flight a bit later that day. [BTW, it was the day before Christmas and the destinations in question were MSP or HNL, where MSP was experiencing a blizzard. The new FAs were very good sports about the change.]