I don't know what restaurants you frequent, but I can't think of anyone being okay with bringing outside meals or beverages. Probably you are just not kicked out because staff is not noticing or apathetic.
I only know of the concept of Corkage, where you pay the restaurant a fee if you bring your own wine to drink:
> A corkage fee is the price charged to guests who choose to bring their own bottle of wine to a restaurant. Corkage fees usually exist at restaurants that already serve wine. The practice of allowing guests to bring their own wine is considered a courtesy to guests. … The average corkage fee ranges from $10 to $40 per bottle but may be as high as $100 or more.
>Or you know, they don't care as long you're a paid customer.
Some do, some don't. In my experience, Starbucks couldn't care less as long as you buy a coffee (and even then couldn't care less. maybe a restaurant would care more but it's on bar logic: they don't mind lending seats until paying customers come in). Smaller shops tend to be more strict about doing that kind of thing.
Apple prevents that I bring an outside donut from competitor's, while Android allows it. That is basically the difference.