> what is stopped someone else from stealing their FOSS (clone and rebrand) and shipping it themselves?

Nothing at all. This is not stealing. The term is "forking." It's how GCC got un-stagnated (see egcs), and how we got the ecosystem of alternate wifi firmware (two examples that come readily to mind.)

This is also where Free and Open source diverge: Open Source lets someone block others from forking their fork, whereas Free Software requires that forks remain free to be forked themselves.