I agree with a carbon tax, but I also think we don't need to go out of our way to support north american oil companies. If that oil gets pumped out its going to get burned as well.
Oil is fungible, but like any commodity it still obey's supply and demand.
allowing a pipeline to be built isn't really "going out of our way". if it were being subsidized, that would be a problem.
but in general, i think it's better to allow a local oil industry, and properly tax and regulate it, than it is to allow all those profits to go offshore to less-regulated, less taxed foreign operations. people aren't going to stop using oil just because they can't get the locally-produced stuff.
The keystone XL received investment and loan guarantees from government.
Beyond that there's a lot of goverment effort that's needs to happen to "allow" it to be built. Securing right of ways or emininent domain on land it passes through.
Oil is fungible, but like any commodity it still obey's supply and demand.