The same is true of any successful open-source project, isn't it? Almost all popular projects are used by for-profit companies, regardless of who owns the copyright on the code.
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.
It's a reasonable position, but I can imagine open-core projects that aren't conflicted this way, and open-source projects that are.