Sure. These things are products to be purchased, after all. Regardless of how you feel about the content of the books themselves, I'd be extraordinarily annoyed if a company could just edit on a whim the content I had paid for and expected to have in perpetuity. That said, you should never realistically buy anything from an online publisher that doesn't let you save a static text copy of the book as a PDF or other file offline. More generally, if you want to buy a book, the best thing to do is to buy from a used bookstore, if you're able. Not like Amazon needs any more money.