I think you misunderstand what CC-BY-SA means. To summarise, content that we, as subscribers, submit to the site is covered under the CC-BY-SA license.
You are free:
- to Share — to copy, distribute and transmit the work
- to Remix — to adapt the work
- to make commercial use of the work
Under the following conditions:
Attribution — You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work).
Share Alike — If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one.
Once you have granted this license, by submitting content to a Stack Exchange site, it cannot be revoked.
Also note that the section of the Terms of Service that you quote:
Other than as expressly set forth in this Agreement, Subscriber may not copy, modify, publish, transmit, upload, participate in the transfer or sale of, reproduce (except as provided in this Section), create derivative works based on, distribute, perform, display, or in any way exploit, any of the Content, software, materials, or Services in whole or in part.
is part of the Network Content section, not the Subscriber Content section, where it says:
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license. You grant Stack Exchange the perpetual and irrevocable right and license to use, copy, cache, publish, display, distribute, modify, create derivative works and store such Subscriber Content and to allow others to do so in any medium now known or hereinafter developed (“Content License”) in order to provide the Services, even if such Subscriber Content has been contributed and subsequently removed by You.†
Network Content is what Stack Exchange, its employees and agents create, Subscriber Content is what you and I create as users on these sites.
What you missed at the start of the Network Content section was:
All materials displayed or performed on the Network, including, but not limited to text, graphics, logos, tools, photographs, images, illustrations, software or source code, audio and video, and animations (collectively, “Content”) (other than Content posted by Subscriber (“Subscriber Content”)) are the property of Stack Exchange and/or third parties and are protected by United States and international copyright laws.†
† My emphasis.