I've only played with Aussies- the less comfortable they are with role-playing (lack of experience mostly) tend to focus more on the more easily understood rules side of things. But that's just Oz so that rules any direct observations re US vs Europe of mine null and void... however a sidenote that may contribute somewhat:
I recently read a short book (30 pages or so) called "A Xenophobe's Guide to Americans". Please note: Written by an American, and not anti-American at all! Observations ranged over the good, the bad, the ugly and the factual aspects of American culture. Generic, obviously.
That said, it was quite confident in pointing out that Americans are competitive. That winning means a lot- a quote being "It doesn't matter if you win or lose, as long as you win" kinda thing.
To me it makes sense that if that's true, then although a (dare I say it) more mature group would consider "winning" to equal the group getting together and cooperatively having a great time, a less mature group (American or otherwise) would consider winning to be beating all the monsters, getting all the treasure, having the most power- at the other characters expense if need be.
The rules can QUANTIFY that version of winning- so putting it all together, I would not consider it unlikely that there would be a slightly greater percentage of American gamers who focus on the rules as it makes it easier for them to feel they have "won" the game.
The evidence I have to back this up? None- it's purely speculative.