The answer is money. These foreign grads come from countries that are flush with oil money and their governments pay Canadian hospitals to train them, and get more for doing so then the government pays for domestic residents. The result is a win-win for hospitals here. They get labour of extra residents and make a profit. These residents typically go back to their home countries afterwards.

At the same time, they do use training capacity that could be used for domestic grads. However the government is reluctant to fund additional spots because they cost money. The government would also have to open more med school spots which also costs money. The losers are Canadian grads and the public since we train less doctors that stay in the country.