Well, it's a "free if you're not generating revenue" model which is similar to JetBrains' recent "free for non-commercial use" releases of their IDEs, and I believe Docker does something like that too.

And famously WinRar which will nag you to upgrade every time you open it but doesn't actually force you to buy it, but expect enterprises will if they don't want to risk lawsuits.

But why call it open source if it's no longer open source?

It is open source. The code is still available under the same license as before. They are just charging for binary builds.

Oh ok, I had misunderstood then.

They dont charge for the source. They charge for paying more attention to your issues, and for compiling the code for you. If you compile it yourself and do some of the work to resolve the issues (planning or an outright PR) then you wont pay anything