Maybe they're selecting for intrinsic motivation. People who enjoy programming to the point they do it for fun, not just because it pays.
Free software work doesn't imply we work for free. We work on our projects, the stuff that we actually enjoy working on. Nobody is going to work on corporate products without adequate compensation.
"Nobody is going to work on corporate products without adequate compensation."
I guess there sadly are many nobodies who do this to hope to become somebody.
If the open source work is part of a hiring pipeline, sure. Contribute to some repository and have it serve as a resume that gets you hired is also a form of compensation. If the work is also enjoyable, then it's a win either way.