That's not really true. They both do it so that they have the ability to relicense the code to their ends and defend any legal challenges against the code-base

That's not really true. They both do it so that they have the ability to relicense the code to their ends and defend any legal challenges against the code-base as a whole. If MySQL just wanted to be able to use the contributed code they could simply require that it be under a less restrictive license (BSD, MIT, etc.)