If you read blogs like "not always right" you'll find that most food service employees are not on board with the customers sitting around and taking up tables. They take it as a personal attack on their income and are very rude about people who sit there.
Even if the management isn't against it, the employees are.
This is a direct consequence of how food service employees are paid in the U.S. They have an income of next to nothing and depend on tips.
Tips do not play such a large part in (most? all?) of Europe and are smaller than in the U.S., so customers that stay longer mean less work without too much pay reduction.
At least that is my pet theory. I only visit the U.S. every couple of years and this year I noticed that the "standard" tip crept up to 15%-20% instead of the 10%-15% that I remember. Seems that employers are paying food service workers even less of a wage than before.
Even if the management isn't against it, the employees are.