Nicecast can be used in many ways, from streaming music you created (and to which you own the copyright) to running a call-in show. These types of streams have no fees associated with them whatsoever.
You can also use Nicecast to stream copyrighted audio. This use case may have regulations and minor fees associated with it. Please see this link for more details on current US webcasting laws. If you're planning a commercial station, you'll want to research your local laws.