It's to keep the Shoutbox from being overrun by them. Most of all the Shoutbox is a place for having conversation - and it's hard to have a conversation when someone keeps posting the same ad every few seconds.
There's really no need to advertise more often, anyway. Within a thirty minute time period, you're mostly gonna have the same people watching the Shoutbox. If those people weren't interested the first time they saw an ad, they're certainly not going to be interested the fifth or sixth time.
So you wait thirty minutes so that the next time you post your ad, some new people will be in there to see it. :3
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