NYT says nobody is sure why, but I thought the general consensus is that due to baseball Americans do hockey the way they do.
A couple of the more serious/skilled hockeyers I've known actually learned to play Canadian style for more precise control. And another guy who was a big dman stayed American style because he was able to shoot a lot harder that way and wasn't worried about precision.
I am American, right handed and played hockey as a left shot defenseman. It gave me better opportunities to play as one of 2-3 lefty shots on the team early on. In Canada players are taught to use their strong dominate hand on the butt end of the stick for better control. In the US baseball dictates the opposite. as a youngster I played either hand, but when we played in my basement, there was a staircase that inhibited the right hand from one side. When we played there, it was always one on one or two on two with a small saw horse for a goal and no goalie. As the home guy I always let my friends play on the easy right side, and I played the left side left handed. I found that I could handle the ball better that way. My shot developed later when I was already switched to lefty. When I played ice hockey, I was the only lefty shot dman and had a pretty good slappper, so from my second year on, I started to get more pp time. Smart move to play lefty in the States, however, that RH shot is better for the NHL due to numbers.