Canada's prime minister is joining the fight against ISIL. If you're wondering about the difference between ISIS and ISIL, here's the thrust.
I would say both are valid. For the people that want the absolute correct meaning, then use ISIL. For the majority of Arabs, Al-Sham is Syria+Lebanon (north of Palestine), so ISIS is okay.
The theory about Obama not using ISIS for political reasons is possible, but maybe too late as the attacks in Syria have started etc.
They should have brainstormed how to call the group publicly from the beginning :D