Except not ... the cost to build was substantial. He had to educate himself how the machine and the OS work in detail. That is time and opportunity cost.
I found it amusing that customers were annoyed with the author for charging for "nothing". If the author didn't solve your problem, or if you could have done it yourself with no help, why did you buy it?
I think an interesting implication of this is that people may feel satisfied having purchased something useless that seems to have enough "stuff" in it.
Also the same old argument used to justify piracy, that it costs next to nothing to copy bytes. And that the product is so not worth the price that those who pirates "wouldn't have bought it anyway", yet there they are using the pirated product, movie and music.
It's because people mainly perceive marginal costs in the goods they buy, and they don't consider fixed costs. In extreme cases like software, news, etc, they think "this extra copy didn't cost anything to produce, therefore it should be free", even though the initial fixed cost was substantial.
This leads to some things like perceiving less competent workers' time to be worth more than more competent ones' (because they take longer to do a task, therefore they should be paid more), etc.
I don't agree this was the case here. They were perfectly happy to pay for a copy of the software, even though making another copy - as you rightly point out - costs nothing.
They were annoyed because nothing had actually been copied. It's like paying for a product and getting an empty shipping box in the mail.
