Is Taxation a legal theft?

Yazad Jal at AnarCapLib compares Customs duty collection by Govt. to highway robbery! I lean heavily towards being a libertarian and hate ubiquitous Govt. However, this comparison really shocked me! At a minimum libertarians are supposed to believe in sanctity of property rights. Highway is a Govt. property and hence levying Customs duty is within Govt. rights. If the highway had been a private property and the owner had levied usage fee or tax on goods then it wouldn't be called a theft. Then why would it be a theft if owner happens to be the Govt.? Just like any other private entity, Govt. had built the road, it maintains the road, polices traffic and so on. Highway robbers don't build road, nor do they maintain them! They occupy a Highway by force and demand ransom. Thus, Customs duty collection by Govt. (on Govt. built roads) is not a theft.

The issue, separate from above, is public (or Govt.) ownership of roads (or other infrastructure). I believe even basic infrastructure can be built, owned and operated by privately. Govt. unnecessarily monopolizes its role in infrastructure business. This reduces the amount of capital available for infrastructure and causes substandard and inadequate infrastructure. It will be better if Govt. focuses instead on making sure that private owners of infrastructure are able to charge and collect users of infrastructure! We should never confuse the two issues.