Of course, Joe Carter is an American and the walkoff home run was an American feat, so it would be kind of odd to have a bias against that - he just happened to be playing for a Canadian company at the time. Nobody complains about Dennis Martinez setting the record for most wins by a Hispanic pitcher, even though he never played for a non-American/Canadian team.
My favorite example of this type of thing was a wrestling match about 15 years ago. Fans were chanting U-S-A! U-S-A! to support a Canadian wrestler (that everyone knew was Canadian) when he had a match against an American portraying a Japanese wrestler.