I find it unlikely that people who pay for technical books were ever going to be your customers. There could be other explanations - maybe once people could flip through your book without paying for it they realized there wasn't any value there for them? Maybe you had sales for a while and then they dropped off like sales do.

I don't think people have the right to spread work that isn't theirs, but i also don't think you can assume the book will always make the same amount of money, or that book pirates were ever your customers.