It's because of productivity.
The article I mentioned in the document I submitted to the committee cites my co-authored Canadian Journal of Economics article where we looked at the data. We found that temporary foreign workers worked longer hours, were less likely to be laid off and had lower turn over, presumably because they have to go back to their home country if they quit, so the firms get higher productivity from temporary foreign workers.
You are right that there are extra costs, but I suspect it's because of higher productivity.