Shockingly, you don't have to be part of some secret free software in-group to write an article for your blog. So the "GPL advocates" that you "keep hearing from" may be different people than the person who wrote this particular article.
Not that I defend the GPL, I simply don't care about which license a project have but Phoronix is not the league of GPL advocates as far as I understand...
What the original devs need to do is put up their own Amazon listings, which they can legitimately label as "official." That way the "you get what you pay for" types can feel good about spending money, and the cash goes to the developers.
It does make it harder to make money from your work, and it's the closest you can get to that while still using a (well-known) open source license. I have no doubt that lots of people are indeed using the GPL for that purpose; look at pretty much any GPL-vs-BSD debate for further proof.
That said, yes, the GPL doesn't actually make it impossible to turn a profit.
Not really. MySQL was multi-licensed and MySQL AB, Sun and now Oracle can re-license it under any license their clients want, provided they pay the price.
MySQL AB was sold for about a billion dollars, proving you can make a whole lot of money while still writing software that can be obtained through a license that respect the rights of the users.
And then when someone does it, like in this article, there is a big fuss.