Thinking Hard Or Hardly Thinking?

This post from Lawyers, Guns And Money seems so obviously wrong I must be misreading it. The elemental mistake is confusing "working hard" (which seems to be the blog's phrase) with productivity. Here's a sample of the reasoning:

Does it seem plausible to [...] a law professor who is probably paid around 200K a year [...] to do whatever it is he does while performing what is technically his actual job -- that he is "working" five times "harder" (using Wingnuttia's definition of "hard work") than a guy roofing houses in San Antonio in July who makes 40K a year?

What matters here (whether you plan to go John Galt or not) is not how hard you work, but your productivity. If you can do something others can't do easily, you're getting results that are "hard" to achieve. Not too many would claim someone paid (a lot) to play baseball or basketball is working hard, but because they have a rare skill, they can raise a lot of revenue and so they're valuable. Many others (including in more traditional professions whose value to society isn't questioned) are also paid quite well because there are only so many who can do what they do. And once inside that profession, how well you do it, and how much you work at it, determines your relative level of success.

Meanwhile, no one denies that digging ditches is hard work. But many people have your skill, so you're not that hard to replace. It's not a judgment on your worth as a person, simply a measure of how much value you provide at your job (whether or not you enjoy it).

Sure, people can be over- or underpaid, but it's not like the whole system is haphazard.