I have no idea how the US tax system works. But how about this thought: from saying you are taxable if running servers in some state, it wouldn't be a big leap saying "your javascript code runs in browsers on computers that are in our state and you are therefore taxable"...
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.
