Oh yes. Good topic Yahtzee. I've never thought of it as an American phenomenon, as I've not compared the box arts as you did here. Didn't really know the localization differences were this large.
I think the cause of this is that American art and popular culture is inherently shallow and superficial. Combine that with a Public Relations industry (since 1920s) that has always been focused on personal identification. Based on Edward Bernays' and his uncle Sigmund Freud's work together. Due to social reasons, the women couldn't smoke in public prior to 1928. A woman was even arrested in 1922 for smoking in public. Women were smoking at home in private, but never out. And that irked the tobacco giants. Thus, one of the tobacco giants hired Edward Bernays to open up the "second front" of the potential smoking customer base. He consulted Dr. A.A. Brill, a psychoanalyst, to find the psychological basis for women smoking. Dr. Brill determined that cigarettes which were usually equated with men, represented torches of freedom for women. His uncle Freud made had the idea that the cigarette even represented the "penis." Well, that's Freud for you. Anyways, this was all about making the cigarette a symbol of female empowerment. This was very timely when the sufragette movement was on the rise, and women's voting rights were being passed to law. At the 4th of July in 1928, a group of scantily clad models, wearing sufragette headdresses walked down Broadway. Provided by a major photo op, at a given signal, they would light their "freedom torches" which were hidden in cases strapped to their thighs.
What did this have to do with box covers you ask? Well, a potential female customer at this time wanted to BE that woman who looked so self-confident and empowered. She thought that smoking would give her those same qualities.
In the same way, the American box covers give potential buyers the appearance of powerful, butch and/or attractive characters who they can "role play" in the game. You never see a Carl Pilkington on the cover. It's almost always someone with supermodel looks.
Command & Conquer is perhaps the most consistent in displaying shallow box covers, following this tradition.
Aside from being superficial, this sort of collage and face-based box art is also something that would give you an "F" in Layout if you were attending an art school.
http://pics.mobygames.com/images/covers/large/950166468-00.jpg
http://pics.mobygames.com/images/covers/large/1209846792-00.jpg
http://pics.mobygames.com/images/covers/large/1209846792-00.jpg
http://pics.mobygames.com/images/covers/large/1071639147-00.jpg
http://pics.mobygames.com/images/covers/large/1071627111-00.jpg
http://pics.mobygames.com/images/covers/large/1200312976-00.jpg