Only it should be mentioned clearly.
He's not, indeed, but that's why the GFDL was created. [1]
Interestingly, the GFDL is designed to be appealing to commercial publishers. [2]
If you advertise your project by stating you are convinced of the "power of Open Source", but don't make use of that "power", how much credible is that?
No matter if you find this argument convincing or not, it has nothing to do with the difference between Open Source and Free Software. It's just about adverticing a proprietary project with "power of Open Source" versus advertising it with, say, "power of Vim".
However, I'm sure the same people will happily spend $100 for a book that teaches some open-source tool. Or at least they would never complain that there's a price for it.
Why?
At the moment we seem stuck in the middle of this transition and you either pay for a locked down copy or you get the Free copy for free. But Kickstarter seems like it's getting mainstream so the concept of paying in advance for something that is built to the specifications of the user may at last become common in software.
Many people believe [...] that you should not charge money for distributing copies of software, or that you should charge as little as possible — just enough to cover the cost. This is a misunderstanding. Actually, we encourage people who redistribute free software to charge as much as they wish or can.
In other words, it is important to distinguish paid software from proprietary software, otherwise the situation and arguments become confusing, indeed.