I read somewhere (I cannot recall where) that licensing a work under a Creative Commons license (or any license, really) is like adding a second license on top of the "license" that copyright law grants (i.e. a Creative Commons license grants permission in addition to the permissions normally granted in copyright law). For example, people are allowed to use your work under the fair dealing exception to copyright law, and Creative Commons licenses cannot do anything to restrict such usage.
The CC BY-SA 3.0 license states in section 4(b):
You may Distribute or Publicly Perform an Adaptation only under the terms of: (i) this License; (ii) a later version of this License with the same License Elements as this License; (iii) a Creative Commons jurisdiction license (either this or a later license version) that contains the same License Elements as this License (e.g., Attribution-ShareAlike 3.0 US)); (iv) a Creative Commons Compatible License. If you license the Adaptation under one of the licenses mentioned in (iv), you must comply with the terms of that license.
But then I could conclude that this means that an adaptation of a CC BY-SA work that is licensed under CC BY-SA is technically dual-licensed under "copyright law" and a CC BY-SA license, which would violate section 4(b) which states that a CC BY-SA work can only be licensed under the same license or a later version. Obviously this conclusion is not correct, as there are many people who have created adaptations of CC BY-SA licensed works without a problem.
Am I reading too much into the license, or am I somehow misunderstanding copyright law and how it works with Creative Commons licenses?