If they're worried about cloning, I don't see how making it proprietary will prevent Prusa from being cloned.
https://blog.prusa3d.com/the-state-of-open-source-in-3d-prin...
One of the big takeaways from this for me is that Jo doesn't really understand IP law. He talks about wanting a new license for open source hardware; the trouble with that is that a copyright license does not and cannot protect a hardware design. That's what patents do.
That's not a defense of current IP law, just a statement on the reality of what it is.
Going non-free is going to lose him a lot of goodwill, just as what happened to makerbot.