You're not seeing the whole picture: I'm paid by the government to do research, and in doing that research my group develops several libraries that can benefit not only other research groups, but also industry. We license these libraries under MIT, because otherwise industry would be far more hesitant to integrate our libraries with their proprietary production code.
I'm also an idealist of sorts. The way I see it, I'm developing publicly funded code that can be used by anyone, no strings attached, to boost productivity and make the world a better place. The fact that this gives us publicity and incentivises the industry to collaborate with us is just a plus. Calling it a self-imposed unpaid internship, when I'm literally hired full time to develop this and just happen to have the freedom to be able to give it out for free, is missing the mark.
Also, we develop these libraries primarily for our own in-house use, and see the adoption of the libraries by others as a great way to uncover flaws and improve robustness. Others creating closed-source derivatives does not harm us or anyone else in any way as far as I can see.