Nexus 4 purchase - Why the tax charge?

First, I'd like to start by saying that I was one of the lucky few that managed to get a Nexus 4 (16gb). And I really want to express my sincere sympathy for my peers here that had their phone at the shopping cart only to see it vanished. I'm crossing my fingers you get to grab one soon in the following days.

Now, as for my question, with the exception of the state of California where Google, Inc is head-quartered:

Could someone please explain why does Google get to charges us a tax over our online purchases?