US office culture is more of a free for all than a lot of places, for good and bad. You don't have unions telling the Type As and workaholics to back off and stop working so much, so they're free to do so, and earn that sweet sweet overtime. The flip-side is when one of those types ends up in charge, there's not a lot to stop them from putting those expectations onto the people below them either. Which you end up with can vary by industry, company, and even division within a company.