I do not think that signature campaigns are worth it for newbies to be honest, They will earn almost nothing and the most are also not knowing what they are doing, plus the fact is that it mostly will be spam.
Yup, newbie's maximum earning per day will be just some post of a higher rank members. But I think the reason why the rate for newbies account are considerably low is because if they did a higher rate for newbies, some people will just make a lot of dummy accounts and enroll on that signature campaign. Probably will just post spams.