This is the main reason for using proof messages.
I'm pretty sure the main reason is bumping and spamming the thread. Otherwise it could be handled by PM, or via the user's Bitcointalk Profile.
Thank you , Our Bounty team is now working on checking all valid participants. This is the reason why Authentication post is required. Ensuring that no cheaters will be accepted on this campaign
Why don't you let them post their address in their profile? You can even use Google Docs spreadsheet to scrape the data, so they'll be instantly accepted or rejected after joining. You'll save yourself a lot of time that way, and reduce spam on the forum.