I'm with John -- I buy it because I want it. I'd certainly take a free copy of a book I want to read if I can find one (legally), but if I want to read it, I want to read it -- so if there's no free version available, I buy it. Reading is my one vice (I've been known to read to excess, perhaps), and I like to indulge it.
When I can't get a copy I need for my book club by any other 'free' means - borrowing from another member of the club or from one of the numerous libraries I belong to. If it is one of my favourites and I can get it for 99c I occasionally do buy one, but very seldom do I buy books for any other reason.
None of your options seem to apply to me. I pay money for an ebook when I want it....seldom for any other reason.
I really enjoy reading books in series, and have several series going so almost always read what I buy. I also like to read authors whose stories take place in locations where I live or have visited. I try to limit these to 4 per month and fill in with free books for variety and possible discovery of an author I have not read before but would like to read more of. I read the reviews carefully on free books because bad formatting will ruin a book for me even if the story is good.
These probably count as a small percentage of those buying ebooks, although I see it increasing, but I bought two of my textbooks as ebooks this semester. Reason: I had to have them. Why ebooks?: they were cheaper than other formats and definitely quicker to get my hands on.
The books I purchase are usually either from my extensive books wishlist and are on sale or something I want without waiting a long time for it to be free. This is especially true for books I want to keep in my personal e-library rather than borrow from the public library or Kindle Lending program.
I buy a book because it interests me, or it is the next in a series, or I have enjoyed the specfic author before and the new book sounds good. I rarely buy books for the listed reasons. Except the on sale one, plus I like the instant gratification of getting an e book. I will actually purchase a paperback if the e book isn't out at the same time, but that makes me angry. There is an Orson Scott Card book that is already out in paperback but won't be in ebook for a YEAR! This makes me livid and I might not purchase that book just on principle. If an author does not publish most of their books in ebook format, I would perhaps buy it as a second hand paperback, or audio format if it is out in that.
I buy books by authors I have been anxiously waiting to read unless the ebook is more then the DTB version OR if the ebook is released after the DTB. I hate delayed releases for ebooks and on principal refuse to buy them if I have to wait.
I both GET and READ the books I buy because the description looks interesting, or the author is one I've read before and I know I will enjoy their book.