It's called being paid in exposure and or experience.
In all seriousness, many fansub translators ended up working for Crunchyroll once they started doing their simulcast sub releases.
In general people do it because they enjoy it and want to see the series have a good quality release in their language. I did fansubbing for a while starting as an editor and eventually learning how to do everything except timing and translating. While it could be stressful at times (sometimes it's a serious pain in the ass to typeset something, like
Maji de Otaku na English! Ribbon-chan, which had some of the hardest scenes I ever typeset, and editing
Nisemonogatari was one hell of a trial by fire to start out with), it's quite satisfying to see everything finished and released the way you like it. When people praise your release, recommend it to others, etc. it's a great feeling and a nice little bonus.