Dung Beetle Etiquette

In public places there seems to be a big difference in seating arrangements from what happens abroad versus what happens in America. In the U.S.A. a couple can claim a large table all to themselves and no one else will join them, even if there are empty seats at that table. But I noticed in Europe there’s a tendency to nab any empty chair, regardless of how many others are seated at the same table. I don’t know why this is so. Maybe Americans are more reserved and don’t want to intrude?