From the natural right to free speech. Anonymity systems rely on people transmitting particular messages in particular ways, whether online or offline. When someone makes an anonymous statement to a newspaper, they do so with the assistance of the reporter who writes the article -- that reporter is repeating what someone else said, and not publishing the person's name. Tor, anonymous remailers, and related systems all operate on the same principle: by running a Tor relay or a remailer, you are agreeing to repeat the contents of a message without its headers.
"Whether we like it or not, governments are ultimately the overseers that permit networking infrastructure comprising the internet to be built"
True, but only on a technicality -- the government also allows us to have computers in the first place. The truth is that we do not need the government to "allow" the Internet; the Internet would be a lot slower without the government allowing fiber and coax to be put along public streets, but we could have built an Internet of point-to-point wireless links without having to ask permission (using only ISM bands, IR, and optical systems, for example). The government could outlaw such things, but that does not mean that the lack of such laws is an allowance, unless you think that all your rights are things the government allows (how dare you breath without thanking the government for allowing it?!).