I don't think that's something we can settle with "they knew about working below market so they shouldn't complaint".
Knowing about something and considering it fair and acceptable are two different things.
A logical argument would be: "they knew about working well below market so they shouldn't act surprised" (because knowing in advance and acting surprised are contradictory).
But being hurt and complaining? There's no logical contradiction between doing that and having prior knowledge that a choice would end bad for you.
(Besides they also know that for some authors that's not the case, and they could -- even legitimately for some -- think that they are better than them, and deserve the same money).
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.
As with many things, there's a widespread and not entirely erroneous perception that working below market for a while is a necessary precondition to later doing well.
Also what's considered "market rate" and what's in 2016 a living wage have relatively little in common. Seven cents a word is the minimum 'professional' rate for short fiction, which very few people in 2016 can actually live solely on.
Knowing about something and considering it fair and acceptable are two different things.
A logical argument would be: "they knew about working well below market so they shouldn't act surprised" (because knowing in advance and acting surprised are contradictory).
But being hurt and complaining? There's no logical contradiction between doing that and having prior knowledge that a choice would end bad for you.
(Besides they also know that for some authors that's not the case, and they could -- even legitimately for some -- think that they are better than them, and deserve the same money).