Forums is an interesting example with respect to permissions because it was the first module with a major requirement to distribute EDIT permissions (posting is basically editing) to users.  The piece you're missing is that any user that can post to a forum MUST have edit permissions on the module.  In most cases, that simply means giving EDIT permission to the Registered Users role.  Don't worry, that's the way it is supposed to work in this version.  I believe that the next version will implement a customs permissions grid which was not possible in the version of DNN that the forums was originally built for.

Cheers

Scott Willhite, Co-Founder DNN