From Test-Scratch-Wiki
The rule requiring New Scratchers to wait 30 seconds between comments exists mainly to prevent spam caused by New Scratchers. Once someone becomes a Scratcher, this rule is disabled.
This rule is mainly to prevent spam and flame wars from happening. While this rule may seem annoying, it is an extremely effective shield against spam. As New Scratcher is the rank for any users who have just joined the site, any spambots will be prevented from posting without a 30 second wait. The filter is also a lot more picky, but there is no wait between comments once a user becomes a Scratcher.