There are a couple of reasons, first and most importantly, the game makes use of private messaging, and you must have an account to receive a PM. Secondly, there is potential for abuse on an anonymous account (i.e. someone could come along and vote or unvote using the anonymous name you were using).
Anonymous users may participate in discussion, but to play the game you must be a registered member of DSLreports.com.
We encourage you to register as a premium member (costs $10) to support the site, but a free account is all that is necessary to play Mafia.