In a way they've always been subsidized because nobody prefers WCS. Measures like Keystone XL are just part of the gargantuan efforts that governments and companies make to make them viable.

No one prefers any commodity until access to the commodity is viable

The irony is that tar sands are abundant and can be seen on the surface but are comprised of the most pipe-clogging useless shit (bitumen) that nothing short of an extra large effort would make them viable.

whats worse is that oil sands have some of the highest production costs because you need to add steam and a bunch of other stuff to separate the other junk from the black gold