I agree with you about the human and enviro cost. Maybe they should extend our regulations so that any US biz has to ensure those regs are met as if they are being manufactured in the US.
That said, part of it is just plain old cheap labor due to living costs.
One of my manufacturers make all their components in MX as well. MX is no dummy. Many areas are forcing companies to provide all kinds of stuff as part of their starting up a plant. For instance, they had to provide a certain standard of meals, child care, and even an onsite med clinic for use by all workers and their families. Even given that, they still saved enough money to justify building a huge plant from scratch just across the boarder.
Another of the manufacturers I use has a very similar story, but on a smaller scale. However, the rest of his story is that he is not only saving a crap load of money, but the paint finish he provides is crazy superior to what he can do in his California plant because the material used was banned by our govt in the '60's, or something, according to him. Our regulations don't prohibit a product having that finish, but prohibit the application of the finish to the product.