When anything seems strange in an airline union contract, ask yourself, how would this benefit someone with seniority? By agreeing to not be paid when the door is open, the FA union gets a higher hourly rate when the door is closed. Now, take two extreme examples, a short haul domestic flight with about an hour flight time, and a long haul flight with a 8 hour flight time. For an FA flying a bunch of short haul flights, a higher percentage of their time is spent during boarding and deplaning. The percentage of time spent working with the door open is much lower on the long haul flights. Senior flight crews bid first for flights, so the more senior crews can make more bank under the current arrangement.