Get Away With Charging Money From Running on Top of Linux and BSD Base?

How do companies like Unrayed and Proxmox get away with charging money for running on top of Linux and BSD base? I run three on-rade servers with paid licenses, loved the software, but was wondering. Well, that's part of open source software. You get to reuse it. It's usually the trademarks that you'd not allow to use. So say you wanted to have some sort of distribution that was based on Ubuntu, as long as you don't call it Ubuntu, that's fine because it is open source and the license lets you do it. And there's nothing stopping you charging people for it, asLong as you make the source code available.