The ebooks we work on are often very old, and have been illustrated various times over the years. We don't want to have to pick and choose a single set of illustrations, nor do we want to forced to constantly justify exceptions for everyone's pet book, so we just have a blanket "no decorative illustrations" policy.

Note that this only refers to decorative illustrations, which is not the same as an illustration required to understand the text. As someone pointed out elsewhere, books like Sherlock Holmes or Agatha Christie, which can include narrative-critical illustrations like a map of a murder scene, or nonfiction which can often have relevant pictures but are not "illustrated" in the decorative sense, get to keep their images.