The most obvious reason to maintain the current update model is that it allows the company ultimate control of how your device acts and allows them to attempt to push their products. Aside from the selling side of it, it does, when everything is working correctly, allow them to ensure the best, most up to date, user experience for all users and those they are likely to show their reader. Mostly, it allows marketing types to feel they've made a contribution by trying to push content.
Personally, I think that all the reader companies have missed the mark. Sure push the books, but try to be a little less obvious -they all have options to go to their bookstores if you're in a buying mood. Why not wise up and offer content I won't find in a paper book? Interviews with authors, discussions about particular books -better yet with the authors as contributors, online book clubs which need not be time constrained like a forum where people can "discuss" their views while reading -they already have browsers built in which could facilitate community activities like online book clubs.
They're pushing stuff in a crass manner when with a modicum of effort they could really be making sales.