[dragonwriter]

> I've seen many job adverts that specifically cited "seeing Github contributions" as a requirement.

> I don't see how that is not enforcing free work.

Paid open source work, on GitHub-hosted projects, still produces viewable GitHub contributions. That requirements enforces open source work, it doesn't enforce free work. Like other professions where people are hired based on portfolios, having a portfolio requirement doesn't mandate free work, just work that is not restricted by the client in a way which prevents it's use in your portfolio.

Of course, if your paid work doesn't fit that model, you may need to do free work to build a portfolio.