[gowld]

National borders exist. Why?

The international Internet didn't exist 200 years ago, so there is no precedent on whether it should be permitted to exist.

Unless you are an anarchist, it's not at all clear what the legitimate functions of government are.

[cowl]

National Borders are not there to enforce "thought". They are there to protect Physical Security (and Economic one that can be ported again to the physical Security). Exchange of Information (What Internet is) has always been a borderless thing. To continue your Borders Analogy, Noone will stop you at the border because you have with you a coded paper that the border guards can not decipher, or even worse because in your country (not with your person) you have coded papers that might be used to communicate the same ideas that are "dangerous" to this country. The Role of the Government is to protect society from phenomena that are provenly damaging the society without encroaching on personal rights. It's a balance that needs to favour the personal rights in any occasion because what this government thinks damaging the other one might not, the personal rights are those that are more lasting.