They might have been offended assuming that Englishmen in Germany would have some basic knowledge of German pub culture. So they took you sitting down at that table as a deliberate or clumsy faux pas. But then they immediately regretted their hasty generalization and how cold they were to you once they figured out you were American. And then they had to make it up to you. Because they knew that expecting Americans to have any cultural understanding while in Europe/Germany is completely unreasonable.