We have all been rejected and had to contact a requester to find out why. The requester might say it was their fault and would like to reverse the rejection but they do not know how. In the past, the requester had to contact Amazon and appeal to them to reverse the rejection, now they can do it themselves if they know how. FRB wrote a blog post listing the exact steps that a requester has to take to reverse rejections. http://turkrequesters.blogspot.com/2012/09/how-to-unreject-assignments-on.html You can now show them how to reverse your rejections step by step with this blog post.
From what I've learned from other turkers here, the requester have to wait 30 days before they can "unreject" the rejected HITs.
No, that is absolutely incorrect. Requesters only have 30 days to reverse rejections, after that they cannot. If the hit goes past 30 days the rejection will stand.