Nothing at all. I'm sure it will happen. If someone forks Chrome OS to add functionality it's very likely it will still be built on Google services. Google isn't interested in developing a full blown desktop OS themselves. If the community wants to add stuff maybe at some point in the future Google will fold some of it back into their official Chrome OS releases. Ultimately it's a product they're giving away for free so they clearly don't intend to make money directly off Chrome OS (or Android) so if you choose to get a Chrome OS variant from another source it doesn't really matter.
What's to keep a third party from changing those restrictions?