What about happening to answer the phone while on vacation? Or perhaps working on a server that's located in another state?
A server is, arguably, a physical location serving products. Javascript running in a browser is the product served. Going with a tried-and-true car analogy, a state can't tax a car manufacturer because the cars drive on their roads, but they can tax them if they produce or sell cars in that state.
What about P2P systems such as Skype?