I think it's primarily associated with Europe because the best European practical pistol shooter (arguably the best period) has always used finger on the trigger guard. I think I remember reading at some point that Grauffel didn't even teach it to his own students though. I have noticed that Lena Miculek has been using one or even two fingers in front of the trigger guard recently. I don't think she used to shoot that way, so maybe she's discovered that it's beneficial for her.