100% of open source devs aren’t giving away the source without a license, since a licence—the grant of permissions for what is otherwise exclusive to author under the law—is what makes something open source.
> That license is how they specify what they want in return.
No, the license is how they legally give away permission to use material that is legally subjejct to their exclusive rights by virtue of creation. The license may be a contract license that, as you suggest, involves mutual exchange of value, but for many (especially permissive) open source licenses it is a gratuitous bounded grant of permission which has limits but does not involve giving something of value back to the creator.
