If the US government, for example, controlled the internet, there'd be no reason why some angry mom couldn't just shout "hey, there's porn on the internet, I demand it be US law that no pornography exist on the internet".
Actually, there's a couple flaws with that argument.
First, if the government controlled it, the First Amendment would apply. Internet Service Providers are free to block sites of rival companies or even censor your personal emails that you send to someone else (which they've been known to do).
Second, there are obscenity laws that apply to the internet. The GOP national platform does endorse enforcing that law and the US government has the legal authority to do so unless the First Amendment prevents them (which it probably does not under the Miller
standard). This would not change if the government directly controlled it.
However, direct government control might at least prevent the issue of corporate censorship. For example, if you sent a message about the Tea Party movement through yahoo mail, it would not have been received by the person you sent it too. But the government could not block it.