I've faced this question before too. I'm sure that people more experienced than me will have a more refined answer, but here's what I've always thought:
Drawing is a "game action," just like writing down moves, disbanding instead of retreating, or (in FTF games) walking away from the board. It does communicate something, but so does supporting your enemy's hold, and so does moving your fleet into Clyde. In FTF gunboat, nobody can stop you from standing up and walking away from the board, just like nobody can prevent you from going NMR in php. And what would be the game result if everybody NMRed, or walked away? Therefore, as "game action" and not speech, /draw should (in my opinion) be permissible.