By posting, you are agreeing to license your question/answer to anyone under the Creative Commons Attribution-Share Alike license. That being the case, SO is a licensee, not the content owner. How can they demand attribution, no follow, etc. Shouldn't the only attribution required for the "clone" sites, be to the original user? I don't understand.
You license your contribution on CC-BY-SA.
This gives the right to StackExchange to to copy, distribute, transmit and modify the work under the same license provided they give you attribution. (BY rule; Done by displaying you name and profile on answers)
From the Creative Commons site:
So you are the owner of the rights and you concede usages on given rules. StackExchange and other sites should comply to this license.
StackExchange sites create another work, based on your primary work. So it's a derivative work. Thus this derivative work is license in the same license (SA rule) you agreed to liberate your content : CC-BY-SA. So if someone quotes StackExchange content, they have to give attribution to this derivative works. Of course, if they are only quoting your part, they can also give credit to you directly for the original work.
Anyway you got the attribution property by transitivity.