So?
Look, in my hub, in my section, there is a nine month difference between my seniority date, and the guy next in line. Twelve other people were hired between he and I, and there was not one of them left working when he started. Four or five just stopped showing up before they even made 30 days. A couple guys made it to 90 days, a couple to six months, and the last one stopped coming a few weeks before this guy was hired.
None of them were forcibly fired.
With the low starting pay, 17.5 hours a week, and no benefits for a year, UPS has problems getting people in the door. They have an even bigger problem keeping them around.
The bar is set incredibly low. In reality... if you can manage to show up with some sort of pants on, can see out of at least one eye, and can get to the facility, you're golden. Show up for 30 days and you've got a job for as long as you can hack it / stand it.
We have a guy who shows up once or twice a week. No one says a word about it. We have a guy who talks on his bluetooth headset the entire night. Again, no one says a word. That's how hard up they are. I'm sure management would love to fire them, but for now, at least they show up and work (sometimes). I don't agree with it, but it is what it is. They have to put up with it as no one else is willing to do the job... at least not for any length of time.
Should folks aspire to have a better work ethic? Certainly. I disagree, however, that what the OP did showed any lack thereof. His sup wasn't asking people to go home because he's a nice guy. Someone had to go home (unless everyone on the sort had their guarantee, which OP clearly did not). He did the sup a favor by not making him force him to go home. It's a win/win.