Right, Grooveshark's entire business model is centered around DMCA protections. Only "user-generated content" affords a DMCA safe harbor defense, so all the copyrighted music they host has to be uploaded by individual users who each assert that they have the rights to distribute the song.
Both Spotify and Grooveshark pay royalties to labels that they have a license agreement with. But if you don't want your stuff on a streaming service, users will upload it to Grooveshark anyway.
This is also the reason why there is so much cool stuff on Grooveshark that is not on Spotify. Forgotten albums, bootlegs, remixes and mainstream music from certain bands. Grooveshark will also never show you a "this song is not available in your region" message.