In my air & ground building, and this is pretty much the same for all our sorts as I hear, the option to go home is given departmentally. For instance, there may be two departments next to each other, with one full-timer and twenty part-timers (and all fters in our sort are 15 yrs+). #1 guy is in department A and he wants to go home to his wife because their accountant is on the phone and they desperately need a missing piece of paperwork for their tax form, but a few people don't show up so he doesn't get the option to leave despite asking his PT Sup. #2 guy, with a year less than #1, is in department B and everyone shows up and manager is screaming on the radio to reduce staff and increase the package count per person over the existing illegally-high numbers, #2 is gonna stride on out of that door and #1 is going to spend the day wringing his hands.
The local doesn't seem to care to get involved and some have said it's specifically because everyone knows how heated the opinions are on the subject. "No old timer should ever leave early!" "I should be able to leave early if I want to!" and then you end up with the union saying, "Look, you guys pay your dues no matter what decision we make, so our decision is to let you continue your bitter resentment and petty feuding. I mean, the company wins here because our position is weakened, but we only care about your dues."
I have a FT coworker who likes to leave early because he has a second FT job that often has meetings and appointments at odd times. He has no issue leaving early because he has 27 years of smiling at management under his belt. I don't really see anything wrong with this personally. There aren't going to be more full-time jobs just because old timers stop leaving early, soooo... the arguments don't really effect me.
