They don't have to give back if they don't use those bindings. They could use someone else's, or write their own.

They could also try negotiate an alternate arrangement, some authors may be willing to do that (assuming they have relevant permission from other contributors if it isn't a solo project, of course).

The example you give seems to be the unreasonable one IMO: “our thing is bigger, so we expect to use your smaller thing how we want”.