Take the example of an immigration officer who receives an application from a young American who wants to come to Canada to work as an unskilled worker in the construction industry. This is someone who speaks mainly English, possibly French, but someone who speaks one of Canada's two languages. It is someone who has worked in a fairly highly developed labour market, who knows about labour standards, who probably has sufficient resources to go back home without someone else's support and who could create a support network here in Canada. When this young American's application is assessed, even if there is information to suggest that there is a risk of the worker being exploited, for example in the construction industry, the officer may decide to issue the work permit anyway, given that the person's individual characteristics indicate that he has the resources he needs to make his way.
But we can imagine a different situation. Imagine the case of someone coming from a country where they don't speak English, who has little education, who has no support network in Canada, who does not have a lot of personal resources, who wants to work in an industry where he or she does not know either what the employers' responsibilities are or what the workers' rights are, and so could be fired by the employer. Admitting that person would probably be more risky than admitting the young American.