It's the server flood mechanism...
To explain this further, the "30 second rule" isn't there because they think you need to read your messages more close up. It's the same reason as for not having a "refresh" button on the arena list. Keeping you from posting for 30 seconds spreads the loading on the server and reduces the chances that a billion requests will all happen at once. It discourages sending a mass number of posts or messages (pretty much preventing advertizing spam since it becomes much less efficient) and helps get rid of the use of automated posting systems.
So, it really isn't there just to nag at you. It keeps their servers alive and the forums tidy from the guys trying to sell you the "male enhancement" tablets from Brazil.
(I'll apologize now if the tone of this post is off in some way; I'm trying to wake up after a couple hours of sleep and don't have any sense of it yet.)