what does "room available" mean?

"On an American beach," says Aronson, "three people arrive and sit as far away from one another as they can. As more people arrive, they fill in, leaving space between strangers. On a Greek beach, three people arrive and immediately sit close together. You can see how Amercians would be annoyed by a family of Greeks setting their towels down right next to them with 'room' available!"

In the last sentence, what does "'room' available" mean?