Actually, they do have obligation, legally, to their users. We all know they can comply to DMCA takedown notices to gain legal protection. However, to gain legal protection from their users , MySpace would be required to put the content back up if a user contests a takedown, and then be able to provide the issuer of the takedown notice with that user's contact information.
Specifically: "OCILLA (the part of the DMCA dealing with takedowns) also includes a counternotification provision that offers OSPs a safe harbor from liability to their users, if the material upon notice from such users claiming that the material in question is not, in fact, infringing."
Logic doesn't apply to the real world. -- Marvin Minsky