I read an interview with someone at Adobe a long time ago that talked about how they knew very well Photoshop was the most pirated software on the planet at the time and in fact they even did their best to track how much it was pirated and factored that in to their estimation of prevalence and reach.

I’d be surprised if the publishing industry doesn’t do something similar in order to get an idea of just how popular a title and/or author is, which will of course factor in to any decision to keep publishing new work by an author and thus make them all more money.