In a company he'd get paied for working outside his job schedules. And if the market could find the right price for something, capitalism wouldn't be such a fail.
And when you're not looking or listening, they turn right around and pay people exactly what the market will bear and pocket the difference. (At least, to your way of thinking. That's not really the best way to think of it, but for rhetorical purposes I'll accept your way for the sake of argument here.)
I feel genuinely bad for this person, who sounds very good at what is actually a very hard job that even a "mere" Physics PhD is not adequate qualification for (evidence: many of them still suck at teaching physics).
But I still can't help but read the complaints about the coldness of capitalism here with a bit of a raised eyebrow. This is a University. No excuses! If there's any entity anywhere that ought to be paying what the professor "the right price" without reference to "cold" market facts right now, with no prompting, no public campaign, just doing it, it's a University. Stop talking and show me how much better your way is than capitalism... but, oh, wait, it's apparently just raw capitalism with a wafer-thin moralistic sheen drawn over it.
And no, there's nothing even remotely exceptional about this one case. We've had a stream of articles on HN that establish how this is absolutely the norm across the board, so no escape out that hatch.
Perhaps those of you who have gotten your ideas about capitalism that ultimately source from the University system ought to reconsider whether your source is that reliable, if they can't put their own ideas into action.