Let us set aside the environmental issue of whether or not the extraction of oil sands is too dirty, and too energy intensive a process, to make sense. If Canada wants to extract petroleum in such a fashion, that is their business.
The question the President faced was whether or not to build a new pipeline to bring the oil so extracted to refineries on the Gulf Coast. That proposed pipeline would have passed through what is known as the Ogallala Aquifer, which supplies water to much of the Great Plains. One accident, and the result could be devastating. Ask yourselves this: Do you like grabbing a burger at McDonald's every once in awhile? A poisoned aquifer could result in no corn, and no corn means no beef, and no beef means no burgers.