There are two meanings that I have encountered for this saying. The most common is to run away or escape. "he made for the hills after robbing the bank" . The hills are not literal hills, it is just a saying. The saying implies great haste. In the context that you are asking, the saying just means that the Americans went up to snow covered hills so they could go skiiing. The sense of the sentence indicates that the Americans went skiing in large number and were very enthusiastic about it.

Michael has the better answer.

It's usually said "Head for the hills." It means these people suddenly got to know the thrill of skiing and took off or headed right towards the hills to do some skiing! The expression "Head for the hills" means to get away quickly. For example, "If this alarm goes off and the cops might come, we should head for the hills!" In your example, it means to get away quickly to the hills for skiing.