mikey, have you never seen the coach passengers get off the coach, use the loos and then have their picnics? I agree lots of people do spend money but many that use the facilities don't.
By law the service stations have to provide free toilet facilities 24 hours a day, 7 days a week, without exception. The operators also have to maintain the slip roads and car parks - in many countries the government own and pay for these essentials including the toilet facilities and rent units to individual businesses for food, shopping etc.
You can be certain that you will get fuel and be able to use the toilet at 2am on Christmas day - will your local garage provide the same?
You would need a brass neck to park up at the off motorway garage, use the loo, let the dog poo, eat your sandwiches and drive off without spending anything - if it's open.