There is exploitation of H1B workers but it's not at the hands of the tech giants. Pretty much all the abuse of foreign workers on visas I've seen is at the hands of these Indian bodyshops that, given the long wait for green cards for those born in India, such jobs are tantamount to indentured servitude.

Speaking as a Xoogler who was on a visa and got a green card sponsored by Google, from my own experience I and others in my position were treated very well.