I would imagine that the majority of those "grannies" started off downloading ebooks "illegally" to get the ebook versions of the books they already own. Personally, I don't have any problem with anyone doing that for that reason (no need to condemn my morals - consider it said and just move on
). Once that was done though, and the expertise was in place and the source sites discovered, the realization that "hey, I can get other books this way, or subsequent books by my favorite authors this way too" made it very easy to take that next step into full-blown "piracy".
Especially if, as was mentioned, the "granny" was already used to mixing music tapes, taping shows off the TV, downloading from places like Napster and Audiogalaxy in her misspent youth. All she has to do is take her peg leg, parrot and eyepatch back out from the box in the attic and she's set to go.
(Besides, don't you just love how it's labeled "piracy" when common folks do it, and just "good business practices" when companies rip off their customers or the government, or when the recording industry rips off their artists?)