Its true that you get BAS (Basic Allowance for Substance). It isnt much, $250 for an E-4 USAF. You can opt to eat at the chow hall, but it isnt free. Eating at the chow hall for free would be Airman that live in the dorms on base. Also, eating there gets VERY monotonous.
Your money tends to go toward phone bill, car payment, food, credit card payments, cable, etc. The same stuff you would usually spend it on in the civilian world.
Also, an E-9 (Highest Enlisted rank) with 20 years of service is only making $4945 a month before taxes. The "$40,000" you quoted atop is quite absurd.