Yes they can, but most will not if they are not busy, but just to be courteous, you should order something at least a drink because you are taking up space that could be used by some one who is going to order food.
Yes If it's like a fancy restaurant but if it's McDonald's and you just walk in to potty and leave or use their wi-fi, nobody really cares. And if they make you leave, they have got to be the fanciest McDonald's ever.
Well of course or all the homeless bums would just move in to every 24 hr restaurant in the world. Oh wouldn't that be nice. Then people wouldn't want to eat there. We would just have a bunch of McDonald/ homeless shelters everywhere