[hydrogen18]

In actuality, it is one of enforcement. It is relatively easy to track and identify water usage. You can disguise a well only for so long. If the US federal government had the means to regulate air usage by so many cubic feet per year, they would.

For some industries this already exists in the form of the regulation of emissions. If you're capping an industries carbon output into atmosphere what you are really capping is the mass of oxygen they can consume from the atmosphere. If you do the thermodynamic analysis of it, you're basically putting a cap on the amount of energy that a business can acquire from using the atmosphere. Most industries run 24/7, so it is really easy to calculate how much oxygen they'll use each year and how much carbon they'll emit into the atmosphere.

It isn't so easy for individuals and smaller businesses. For example, I own a generator that uses oxygen from the atmosphere to get electrical energy. However, I only use it for a tiny fraction of each year.