@Ross at Play
It sounds possible in theory, but I suspect that is very rare.
I'd say that human nature results in some who are willing to do one, and some the other (download illegally or pay for downloads) but almost none will sometimes do one and sometimes the other.
Ordinarily, I'd agree with you. But these sharing sites are not just populated by thieves, but often by people who simply believe in the free flow of information. I know, because I've participated in them many times myself. But instead of just blatantly uploading whatever I could lay my hands on, I'd work to find the difficult to locate obscure stuff, long out of print, and offer it for those looking for particular types of stories.
I too, believe in the free flow of information, and think the newer 75+ beyond the death of the authors is much too long to restrict people's ability to read a story. Many people now view the internet, as a whole, as a vast 'personal library', where everyone is free to share information.
Those people can well afford to purchase books they like, and often do, just to support the artists in question, but after the artist has already earned millions from a single piece, figure they've already had their due, and it's time to let the work live independently from the author's dictates.
And strangely, even though I'm protected by those same copyright laws, I can see that particular logic, and largely agree with it.
For a long time now, many authors will freely offer their own books up to torrent sites, using false names so no one will suspect that they're plants, as a way of promoting their own works, and they (reportedly) increase their own sales by using such tactics. I've never tested it myself, but I'm now thinking the results of my 'experiment' in giving books away for free weren't the result of my experiment, but are actually due to someone distributing my books for free. Granted, 28 books isn't much for the thousands being shared individually, but it's 28 more copies than I'd have sold otherwise, which benefits me, as well as the thousands participating in such sites.
As such, I'm still unsure how to feel and respond to such sites.
In my own case, most of those I helped were trying to access books they couldn't access legally (most notably, books in Dutch of famous American novels). Those were hard to come by, and those people would have gladly paid for them but were unable to.
And I'm a living embodiment of what you describe as being so rare, someone who both read something for free, and then went out and paid money for (though granted, not for the same books). Typically, I'll read something I ordinarily wouldn't have read, but once I did, decided I was interested in the author, so went out and purchased more of that authors works.
So there are those us us who operate in that environment. How many of us there are, though, is an open question.