> Absolutely nothing about free software requires or even implies any responsibility to “give back”

You're correct about that. The free software itself doesn't confer any responsibility. But the free software exists inside other contexts. Social/moral context. There're also future contexts for you or humanity. For example, if developing free software proves to be a sustainable model for people to do, you might get other projects LIKE the Blender Foundation to crop up in the future. You might benefit from them directly, or benefit from them by enjoying the things people produce with them. Also, if it's a tool that you like to use, maybe you just want that specific tool to continue to improve.