IIRC, this was fixed in the last few releases- it had to do with OS X's weird user-session/permissions management. At the very least, I was able to comment out the following workaround:

    bind-key -T copy-mode-vi Enter send-keys -X copy-pipe-and-cancel "pbcopy"
    bind-key -T copy-mode-vi C-j send-keys -X copy-pipe-and-cancel "pbcopy"
    bind-key -T copy-mode-vi MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel "pbcopy"
    bind-key -T copy-mode-vi A send-keys -X append-pipe-and-cancel "pbcopy"