What is the rationale behind paying foreign workers less?

Paying foreign workers less than Canadian born may cause Canadian workers to see immigrant workers as serious threat of taking away their jobs. Which employer would not prefer to pay less than more? KI don't think this is such a great idea.

http://www.cbc.ca/news/canada/manitoba/story/2012/04/27/mb-labour-foreign-workers-pay-120427.html