I don't think a claim like "the pay for job X is unfairly/unacceptably low" makes sense on its own (or at least, it's not a claim that people would be sympathetic to in general). It tends to contain an implicit argument that either a) job X is representative of the most lucrative (reasonable) jobs available to people in particular circumstances or b) job X is somehow socially valuable. No-one complains that e.g. surfers are underpaid, and few would be sympathetic to "How much do you value your favorite surfers eating or having health insurance?" I took kevinr to be implicitly claiming that authors are socially valuable, that I would prefer for people like his friend to write instead of working at their market rates.