Canadian oil is piped into the US already. The Keystone XL that they want is a leg to get their oil down to the Gulf of Mexico, so they can sell it overseas.
So to recap. A foreign corporation wants to take Americans' land through eminent domain, spilling untold quantities of poisonous gunk into one of the largest fresh water aquifers on earth, so that they can ship oil through our country to markets overseas.
Asked why they wouldn't just pipe the oil west to their own western coast and refine and ship it from there, Canada replied in effect, "Pfffttt... Our people ain't puttin' up with that shit, eh?"