Some points:
1-You don't boo kids. These players are 17 and 18. What's next, boo the Team USA Bantam team? Pee-wee? Stupid.
2-The USA is arguably Canada's biggest rival in International Hockey. It USED to be the Soviets/Russia, but they have slid in the wake of the break-up of the USSR, and the USA has taken a huge step forward. Remember that the US won the World Jr's in '04, the World Cup in '96, Plus I believe they beat the Candians at the Olympics at some point as well. They've played Canada the toughest in the last 10 years or so.
3-A US player, Jack Johnson, threw a vicious elbow to the head of a Canadian player, Downie, in the dying seconds of their round robin game a couple of days prior, after Downie made a pass for the final goal. It was a real cheap shot that got a lot of press and really had the local fans here calling for a suspension, which didn't happen.
4-There is some Anti-US/Bush sentiment in Canada over the softwood lumber dispute, Mad Cow, and Iraq. Right or wrong, agree or disagree, it exists. It may or may not be more prevalent in Vancouver than elsewhere in Canada, I don't know.
The booing shouldn't have happened at all, as they are just kids. HOWEVER, the booing had WAY more to do with the rivalry and Jack Johnson than Bush, Softwood, Mad Cow, or American people in general. They were booing Team USA, and to a lesser extent, Jack Johnson, not the US as a nation or a people. It is a little ironic that by proxy they were cheering for Canada's old arch Nemesis, Russia, by booing the Yanks.