I think rahimnathwani's point was not that they get extra pay so it's fine, but that it seems economically irrational to overwork fewer staff if it's actually more expensive.
Here in Norway it's similar with doctors, they get paid a lot because they work crazy hours. But the doctors' association is fighting to keep it that way, as the old timers who didn't burn out along the way enjoys the high pay more than their spare time.
It's hard to argue you're underpaid if, as a result of short staffing, you're getting paid more (both in absolute terms and per unit of effort) than you signed up for.