I feel different about fully open source projects. You are working for free for the benefit of the company, all other companies who want to use it, and the rest of the world. I'm happy to work for free when it benefits everybody equally. I don't care that for profits benefit, because they all benefit potentially the same amount. It's a boon for the world.

With open core, you're working for free for the benefit of the leverage of a company to better position their secret sauce. It asymmetrically benefits the company more than everybody else. I only do that when I'm getting paid to do it.