To be in a Signature Bounty Campaign you will need to be able to use a Links in your signature and Jr. Member can't do that.
So you will need to be Member.
You need 60 posts and 10 merit.
Whose jr member can not follow signature campaign? Many projects allow members of the jr to follow signature campaigns. Only part of it that prohibits jr member follow signature campaign in its bounty. Because according to their jr members will only make spam.
Why mate, LOL. I see even you are junior member rank in this forum. I do not accept all the people have the same mind and spam the posts everywhere. Since you do not find the good campaign or seeing the spamming guys only you may see this all junior members are spamming everywhere.
If you want to survive in signature campaigning you need to maintain your post quality better because if manager goes to pick the guys manually then you cannot get inside the campaign mate.