The real reasons this is opposed so vehemently is that the tar sands are refined twice, once at the source to make the synthetic crude and once at the destination. At the source, they have to take what is a waxy sour product and change it to a light sweet product, and the result is a lot of pollution. Yes, its pollution in another country, but some people have realized that it's on the same planet, and might not be a great idea.
ETA: I'm only against this to the degree that I think it's a boondoggle. We're going to have petrol for quite some time, and the deniers are forcing the day at which we can be free of dependence on oil to be AFTER the oil gets so dear that world economies collapse from the cost of it.