Specifically, article VI requires - "the activities of non-governmental entities in outer space, including the Moon and other celestial bodies, shall require authorization and continuing supervision by the appropriate State Party to the Treaty"
Interestingly I don't know of any established processes for monitoring the activities of companies in outer space apart from getting approval from the FAA. It's not really ever been a problem up 'til now.
A tangible way to explore this is to ask "What if a craft, launched by SpaceX from international waters, fell on country X?"
If X is Canada, the resolution process seems straightforward. American and Canadian courts and law enforcement are sufficiently cross-linked to allow the latter to pursue SpaceX's assets and personnel.
But what if X is Iran, or Venezuela or North Korea? We would have to consider diplomatic and geopolitical factors. That looks much more like the anarchic environment of international relations rather than a sovereign-corporate dispute.
International conventions require ships in international waters be flagged. This comes in handy when e.g. a ship needs help or causes trouble. I don't see the extension of this principle to spaceflight as being unreasonable. If Bezos's bird comes down on my house, I'd prefer there exist a venue in which to sue him.
https://www.vice.com/read/hows-the-congolese-space-program-d...
This doesn't seem to have much to do with outer space treaties, unless perhaps your house is on the moon. Assuming that isn't the case, your house is protected by the laws of the nation in which it is located, and perhaps those of other nations as well.
I would imagine it would be the same for any other country that has launch facilities as well.