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.
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.
Of course the newbie will make spam posts that's why they are not allowed in most of the signature campaigns.But the new merit system will take care of everything here after.

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.
Agree with you but if newbies are allowed to participate in bounties then the bounty hunters will make the forum into a play ground by creating thousands of new accounts just to participate in campaign.
But the main reason of this forum is to know about the crypto currencies not just to participate on the bounty campaigns.
That is also a good point if newbie accounts are allowed then there will be lots of new accounts will be registered here then it will make the forum into garbage shit posting forum.