Contributions to StackExchange sites are licensed cc-by-sa.
Are you allowed to revoke the rights to an answer after you've posted it?
There's two things at play here. First, our license - which covers what folks can do with our content and specific rights that contributors retain:
Folks can re-mix, re-use and share our content, provided that they attribute it correctly
Contributors have the right to be disassociated from a work for any reason (a safety net for cases where a collaborative work goes in a direction not envisioned, nor agreeable to a contributor)
Then, you have our terms of service which all users must agree to in order to participate. We address this in section 3 (Subscriber Content):
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.
This is in place because we can't possibly hope to document how many questions and answers we didn't receive because the user posted on the site. We don't know how many people didn't answer a question because someone's answer was there and covered it completely - you can't document a negative.
Hence, in order to ensure that one's departure doesn't leave gaping holes in the collaboratively curated resource, we require that you grant us an indefinite and irrevocable license to display your contributions under the license that we have in place at the time you contribute the work, even if you've deleted the contribution previously.
If a user no longer wishes to participate on the site and elects to leave, we can disassociate them from all of their contributions, and remove any negatively scored posts in the process of removing their account. However, we can not make exceptions to our terms of service when it comes to this. It's simply not fair to the resource as a whole.