Really?

If you google trade unions and migrant workers in about 5 seconds you would find dozens of articles to shoot down that statement too.

http://www.ihrb.org/commentary/guest/trade_unions_building_bridges_to_pr...

I think the problem you are confusing is illegal migrant workers, who don't want trade unions getting involved with their working conditions, since that would bring to light the fact that they are working illegally.