Is it okay that I never order food when I'm out with friends because I know I'll be eating all of theirs?
Why do you eat their food instad of ordering some for yourself?
I eat their food because it's free.
There's nothing wrong with wanting to save a little money when you're hanging out with your friends.  However, what you're doing is excessive.  Eating other people's food without permission is taking advantage of them and I'm sure your friends don't appreciate it.
They don't mind that I eat their food.
I'm sure they're okay with sharing their food with you sometimes, but if you take their food every time you go out then you're being rude. Your friends might be too nice to tell you that what you're doing is inappropriate.