Authors want their works out there. People made music and wrote books long before there was copyright. Even today, there are TONS of publicly available written works of fiction and non-fiction on the internet for free. I won't pretend that all of it, or even most of it is any good, but there are some real gems that people release. Look at even some of the fan-fiction works - many of those are written as "ubers" of whatever characters and have nothing to do with the original work. There are some exceptional ones written for no other reason than because someone wanted to share it.
There are people who get donations for original fiction that is only published on the internet, much like artists used to be commissioned to make new works simply because someone wanted them to continue with their art.
DRM and the whole copyright industry is largely a racket. True artists (and even bad ones) will continue making their art whether they make money or not, because they either want to share, want to improve, or simply because they can.