It's free in the same way that stealing is a way to get things for free. When you pay for petrol for example, the money you pay goes to the oil extractors, refiners, distributors, etc. for the transfer of oil from the ground to your car. But you're not paying the earth for the taking of those resources (carbon budget I can be considered a "resource"). It's analogous to paying for someone to steal for you.

In an efficient market, the price you pay would include a carbon tax paid to the "Earth fund", and the fund would be used to replenish the planet's resources (eg. by carbon capture and storage).

But until then, the version of capitalism we operate is one where the largest participant (planet Earth) is only being stolen from; never being paid.