your analogy only works if the rental company is paying for the gas. You can drive as many miles as you want. You just can't waste gas. They just don't want you lead footing at every stop light and doing 90 down the freeway.
That's why the carpool lane was invented. To show appreciation for those who are gas conservative. Is the goverment discriminating too?
No, in my analogy the rental company is NOT paying for gas. The rental company is offering Unlimited mileage rental car. Meaning you can drive as many miles as you want for no additional per mile fee compared to other rental car companies limited mileage restrictions.
Also, in my state, car poll lanes aren't in existence.