The answer to that, frankly, is pretty straightforward: because that electricity costs money to create. It doesn't just cost money to get to your house, but the source of it costs money. Similarly with gas; you use more gas than your neighbour, you should pay for more gas, because gas is a resource that has value in itself.
Bits, as Matt said earlier, have no value. They're light. They're pieces of air. The value they have is the value that comes with the creation of it by you when you write an e-mail, when someone else writes a story. It has no intrinsic value. The only thing that has value is the pipes, and you pay for that once and for all.