The main reason is the the newbies don't have signature links then how they can participate in the signature campaigns. 
But there are some bounties which accepts newbies too but most of them who accepts newbies are scam projects never pay you at the end.And other reason for that is to avoid the spamming around the forum because they will tend to fill up their minimum quota for their campaign without any knowledge.
But there are some bounties which accepts newbies too but most of them who accepts newbies are scam projects never pay you at the end.And other reason for that is to avoid the spamming around the forum because they will tend to fill up their minimum quota for their campaign without any knowledge.
Signature is the issue and moreover if newbie who does not have proper knowledge promoting one project in the forum no knd will believe that that is trusted one.
Moreover this rule is healthy for the forum to avoid the knowledge less people to avoid the bounty earnings from the ICO project. Conclusion is same your point. To avoid the spamming.