I'm not German but I work in Germany. I work in an two floor office with >50 people; we're 4 people in our room, working more or less on the same projects, as a team, and we all have small children. When one of us is sick, they stay at home until they're not contagious any more. You're not only thinking about the people you're working with, but also on the impact of their health on their family members. This is both because of "common sense" and of a "practical sense": you don't want to infect other people and see them get sick (common sense) and you don't want to have your team mates missing, which sometimes makes your job harder.

I believe this is also related to the culture and the way the employee is protected by the law when sick. In Germany, you're not going to get fired because you were sick for, say, a week. Although I don't know all the details and subtleties of the American Health/Work System, I can say I haven't heard that many great things about it. Which, in my eyes, would confirm/explain why in the GP's example, their American colleagues work even when sick.