Yes, the U.S. does ignore WTO and NAFTA rulings when they are against perceieved U.S. interests. That is one reason why a lot of Canadians, whose livelihoods depend upon the lumber industry, don't like the U.S. They see the U.S. as not abiding by the agreements the U.S. helped to generate. It's a legitimate point in international circles. If you negotiate a treaty, you need to live up to your end of it.
The problem the Canadian sawmills have is we(the US)are their biggest customer. The lumber/osb markets are technical overproduced even with 2 million housing starts. The lumber market was in the tank until Katrina saved it. The Canadians have no where else to go with their product and the US govt. knows it.