Now as far as I know Cab companies are private. I believe many actually work as self employed under the banner of another. I could be wrong as I'm not a taxi-expert. But I am quite sure they aren't paid by the government.
That would give them a little more descretion over who they allow in a cab that they, or a private company owns would it not?
That said, the airport should also be within its rights to refuse them airport licenses if they refuse the airports passengers, and they should probably do so.
There are two types of cab companies. Those with fleets which all have the same looking vehicles and those that employ private owners. The private owners typically have just a placard of the company they are servicing. They also typically do not service airports.
NOW it is interesting that the airport can refuse to allow cabs that do not meet their guidlines. Someone mentioned that the ACLU may call fowl because it is focused on Muslims.
This isn't true ANYONE who chooses not to follow the rules will not be allowed to work the airport. Additionally the airport is not the only place you can make money. Finally you can drop off just not pick up at the airport.
Now private business does have the right to refuse service to anyone for any reason. This doesn't apply here because the airport is the one that is setting the guidelines. Compaire this to Walmart, "If you want to sell on my shelves then you must do x." is the same as this scenario here.
Discrimination by law definition is not the same as the social definition. I'm paraphrasing for I am not a lawyer.
Social discrimination is the process of devaluing someone for attributes and or beliefs.
Legal descrimination is the process of restricting or denying a specific group of people based on a commonality.
The airport is not saying, "No muslims can run cabs!" it is saying, "Those muslims that refuse to meet the minimum set of guidelines for the airport will not be licensesed to work here."
It WOULD be discrimination if the airport said, "Because of the religious beliefs of muslims we believe it makes them unable to perform in the cab function. Therefore no muslims will be allowed to run cabs within the airport.
So I think the issue here is legal vs social.