There's always money to be made from market inefficiencies.
He wasn't honest with the companies that the work product was not his.
When dealing with companies, so many of them strongly specialize in methods of dishonestly that aren't quite illegal and in using information differences to their advantage, purposefully choosing to avoid any chance of disclosure. To that extent, I have a hard time as seeing the morals as questionable and instead seeing it as someone beating companies at their own game, assuming he never directly lied that it was his own work. Companies almost never tell you what they view the real value of your work to be, which is also hiding a form of information that the other party would have found relevant in negotiation the trade of money for work.
I can see how someone who was doing this might excuse it to themselves as simply exploiting a perceived inefficiency in the system, but nobody other than the person actually doing it would buy that excuse. Even the phrase itself (I was just taking advantage of an inefficiency in the system!) is tainted by associations with white collar crime. That's a phrase that makes people sound like they think they've done something wrong and are trying to spin it, even when they've done nothing wrong.