Can't speak for US policies, but in the UK any deportees being removed at state expense are able to claim miles for the flight they are deported. They are not however permitted to UPGRADE these flights, even if they are not being escorted. These are conditions that airlines have to agree to in order to get the UK Home Office removals contract.

The immigration escorts however are able to earn miles on their tickets and always make sure they do so. They are for the most part hooked up with mileage brokers who re-market the miles and give them a share of the profits.