Well the reason is that if the US doesn't give up control, countries have been threatening with building their own internet infrastructure to run in parallel.
Since when was "do what I/we want or I'm going to take my ball and go elsewhere" been a valid reason?
If these countries (Brazil, Russia, etc) did create a "second internet", then Google, Facebook, Twitter, etc, would all be shut off from their customers in those regions.
As far as I can tell, they'll do the same thing once control is globalized. At least now, they can say "fine, we'll make our own Internet" and the rest of the sane world can say "cool, see you later, good luck with your Internet without any of the shit your people actually want because we don't care to jump through your retarded hoops to appease your insignificant ass" (and yes, it really is a matter of insignificance because the shit most of these countries are yammering about and want control for is to further enforce their own restrictions on others, whereas we enforce openness (for the most part anyway, far more open than many of these other countries would have it be)). Besides, regardless of how it works out, we already know most big Internet businesses will do what they need to to ensure their service is still available, but I'd rather that choice be at the corporations level, and not made a requirement at the behest of tantrum throwing nations/governments.
Can't do the math?
They get a lower customer base, lower potential profit, lower actual revenue. Unless the spend the R&D on developing their platform to conform to the "second internet".
Why yes, yes we can, and it's already been done. Look at what Google did with China. We didn't have to give up control of the openness of the Internet to the rest of the world. Let the nations that hate all that freedom build their own fucking Great Firewall and control their people that way. As I said above, if the Internet companies give enough of a fuck, they'll find a way to make stuff work, and that's as it should be, IMO.