Nope, it is evidence that Marx doesn't much understand human nature. I'm not saying anyone has it all figured out, but his is one of the more popular theories that is still somehow so far off the mark.
I think
@SLW's
quick and dirty take on Marx is probably the better interpretation. Marx was a champion of progress; at times he seems to have thought communism would be the almost-inescapable outcome of capitalist development. (FWIW I disagree and crack up when people cite Marx as if he were a prophet.) I also don't think it's possible for us to make objective claims re: human nature. 1. We're interested parties, judging in our own case and 2. it stands to reason that how people are -- what they do, how they think -- is substantially influenced by the world they are born into as (initially) helpless dependents. The point is not to say nature plays no role in how we are, socially and individually, but I don't see how one could isolate a 'true' nature from social, cultural, and economic, etc conditions.
So which is it? Are union brothers so afraid of losing their jobs that they work off the clock? Or is it non-unionized workers. I assert it is just that people are fearful in general, and that gets exploited, whether it be by "evil capitalist pigs" or by failed economic theorists.
The brothers I know who donate their labor say they do it because they want to be involved in their kids' lives. If they had a stronger option than the 9.5 list -- which, as we know, more reliably pads our income rather than lightens our dispatch -- I'm sure they'd use it. However, accomplishing that would require concerted action, and likely not just at UPS, but across the entire transportation/freight industry. It would require challenging the whole notion of 'operational needs' which rationalizes grueling hours for the sake of moving commodities.
As a side note, I have no interest in blaming 'evil capitalist pigs'. That's the socialist equivalent of the Illuminati conspiracy theory. Capital may well be an 'evil genius' but it answers to no one in particular, not even gaudy billionaires. I think of capital as AI or Skynet (lol); it is the supreme agent of history -- we are bit players, and since it is capable turning nearly everything to its advantage, we
mostly do its bidding realize it or not, like it or not.