> There are mechanisms to charge for it.

Only above a certain price point. Our financial system nor our consumer culture support micropayments yet. This effectively limits the web properties that can charge users to a small fraction, so unfortunately "just charge money" isn't the widely-available solution I wish it was yet.

Hell, it's more viable to charge users for your mobile client than the backing service. It's kind of the closest we have to micropayments in a way.