tmux uses a client-server model. This runs into issues on OSX because when the server detaches it loses context within the window manager and then launching (e.g.) MacVim from that tmux session will cause the window manager Copy-Paste to stop working (because the OS doesn't think the process has the requisite access level).

On the other hand, it makes it a lot easier for multiple people to connect to the same tmux session. In GNU screen (at least prior versions), there was an explicit check for 'uid == 0' before allowing some of the screen-sharing features (iirc). This meant that even if you chmod'd all of the sockets/ttys that needed it, screen would still refuse to operate in that mode unless the process was setuid root.