Nothing at all
![Smilie](https://www2.mobileread.com/i/smilies/smile.gif)
They're a free Public Domain site, and in order to keep free, they need your donations, and everyone else's. The same can be said of Project Gutenberg or Manybooks.
*edit* Feedbooks seems to have opened a bookstore of their own... Howevdr, the other two are still donation-supported.