Why do companies open-source their design systems? Other people aren't allowed to create a website that looks like Porsche.com because of copyright and trademark laws anyway.
I always assumed it's because properly securing it is too much work for little gain (because of the reasons above), but maybe there's more to it?
Essentially, there are contractors and thrid parties that develop stuff for them and they want it to look the same as the rest of their stuff. Putting it out in the open made things much easier. The private registry was hard to integrate in our CI/CD pipeline. You couldn't just open an issue on GitHub (it existed, but I think noone was looking at this), but had to write some slack messages - to which you needed access to.
It's just more cost-effective this way.