True. It's a complicated issue:
-are these good jobs for the temporary workers? I think yes
-it doesn't benefit Canadians to have the food grown elsewhere
-on whole food standards in Canada are amongst the best in the world
-labour standards are generally very high in Canada, but I suspect these types of jobs would be near the bottom in Canada (but certainly not globally)
My opinion is that it would it would be beneficial to have more Canadians in these jobs (youth are looking for part time employment for example). But mostly to keep the employers honest.
*edit more thoughts
-it's probably economically beneficial for Canada to subsidize wages for Canadians to do these critical jobs. (Money doesn't exit the county)
-for the same reason TFW are more beneficial to donor countries than immigration.