A printed book requires a considerable investment in printing, binding and distribution. Since costs are high to get products in the hands of consumers, authors have to either spend a lot of their own money to get titles to market, have backers or have to get publishers to be willing to take a chance to print the books. Risk is involved, all parties proceed with hopes of making enough money off book sales to recoup printing costs and, hopefully, make a profit.
The barrier to entry is much lower for authors of eBooks. Using commonly available computer tools it is viable to print and distribute your own titles directly to readers over the Internet. eBooks can be consumed on devices ranging from cell phones to eBook readers to desktop computers. This can be very empowering for niche publications. Yes, Digital Rights management (DRM) is possible with EPUB formatted content, but like open source software, it may be preferable to make money off of free distribution of open source eBooks by making money off of related products or services.
There may be other business models where it could be advantageous to give the books away rather than charge for them. Since the production and distribution costs can be so low, it may be possible to profit through ad revenue. Author/publishers could include pages of advertising in eBooks, targeted to the reader demographics of a particular eBook title. If such a book proved popular it may be possible to gain further revenue by marketing spin-off merchandise.
Imagine, for example, an e-publisher giving away Harry Potter type eBooks and making the money off of movie sales, toys and related goods. That is not that hard for me to imagine.