I will start to answer this question and perhaps someone will want to add something.
There are requirements set out in the program that everyone must meet.
I mentioned, for instance, that employers must first inform Canadians of their needs before they get a positive labour market impact assessment, or LMIA.
They also have to provide reasons for refusing to hire Canadians who have applied. They must explain why they did not hire them. The program framework sets out requirements to ensure that the situation you describe does not happen, but it is not perfect. It is not always like that. There are cases where employers do things they are not supposed to do and it is up to us to try to detect and review them, but it does happen from time to time.