Do you see any reasons why signature campaigns would need to accept Newbies? There are few main reasons why they aren't accept Newbies:
It wouldn't be beneficial for advertisers. Newbies are allowed to have only very basic signatures without styling, links and only up to 50 characters. Such advertisement wouldn't be visible well, so there are no point to hire Newbies.
Limited knowledge. Newbies usually don't have solid knowledge about Bitcoin and cryptocurrencies. It's likely that they will spam or make low quality posts and it would only give bad image to company.
Abuse. If signature campaigns would accept Newbies, what would stop users from making 10, 20 or 50 bitcointalk accounts and join campaign with all these accounts. And now when campaigns accept Jr. Members, it's easy to cheat these campaigns. It tooks only 1 month to reach Jr. Member rank, you don't need any Merit, so, many people make unlimited number of Bitcointalk accounts and then they join bounties with these accounts.