The part of your prejudice which has some truth is that one of big values of the American culture is competition. Sometimes men who are called ¨nice guys¨ are ones who are more cooperative than competitive.  An agreeable personality can be a big advantage and many American men are very friendly and nice in most situations.  Some competitive men really are cooperative as well with members of their ¨team.¨ But in some situations, you will see American men ¨look out for number one¨ (themselves)  which can be more culturally sanctioned in the U.S. than is likely in Japan or other more collective cultures. (Actually this is somewhat true for women as well.  Although traditionally women compete in term of appearance, their home, their children´s achievements, etc.  I would say that is changing as sex roles and parenting are changing a bit.)

There are many sayings in American culture like "good guys finish last." It is impossible to speak for every case but the corporate world is indeed quite "cutthroat" or "dog eat dog" as we say, and I can't say my experience was any different. Politics play a big role in corporate America and society in general, so those that are able to make decisions in "cold blood" and not care about the feelings of others, might indeed have an advantage in American business and society.