Well, the problem with the Tar Sand oil is that it's some of the most expensive, dirty oil to produce in all of the world.

Sweet Light Crude Oil(Saudi Oil, cheapest, best quality, easiest to extract, cleanest oil) can be pumped out of the ground for about $10-$20 a barrel. This means that even if oil dropped to $30 a barrell, the Saudis could still sell oil, and make a profit. However, Canadian Tar Oil(and American Shale... and Venezuelan Heavy Oil) cost MUCH more to produce than $10-$20(it's more like $50).

So, with the current price of oil being around $50 a barrel(as compared to the $100+ when most of these Oil Sand operations were created), there is not much wiggle room for producers of heavy oil.

If, for instance, the Saudis were to flood the oil market, and drop the price to $30 a barrell, Canada's oil operations would no longer be profitable, and they would literally have to stop producing oil altogether.

Considering the Tar Sand oil is already fairly close to the line of being unprofitable... the idea of taxing it seems very dangerous, as it makes the "unprofitable" line move even closer.