Not in all major cities. NYC has placed fuel mileage requirements that are being phased in over the next few years. Currently, the only vehicles that meet the requirements are hybrids. Obviously, if an automaker comes out with a suitable vehicle that is not a hybrid that meets the mileage requirements, that vehicle would be able to be licensed as a taxi.
Really? Why would one buy a new police car, use it for less than 500 miles, then sell it to a taxi company or owner?
In NYC, there is a requirement that all taxicabs have less less than 500 miles traveled at the time of hack-up.
For reference, see Chapter 3, Taxicab Specs, page 3: