this is a myth. I have enough experience with migrant workers to know that they're not stupid enough to work for less than they are worth, and are always on the lookout for higher paying work, and work with benefits.
It's a good thing that the H-B2 thing is one-time, because no migrant that I know would work for less pay than American workers and, though they might go without "benefits," -- useless regulatory overhead, in other words -- they often don't stick around for long in such a place.
This argument is spurious and is actually employing a familiar leftwing device: the premise that poor souls are being taken advantage of (not true); and that premise is being used to push an agenda that really has no concern whatsoever for the well being of migrant workers (protecting American laborers -- a sort of tariff on foreign workers, if you will).