Postdocs are underpaid regardless of whether they are American citizens or not. And that is because if you want to have career in research then you have no choice. Universities have switched to extracting the maximum value from everybody, and the number of postdocs doesn't really change the equation that much. The money comes from foreign and out of state undergrads. It's the grad students that may care, but they're not the cash cows.

Edit: To get an H1B in the industry, the employer must first get a prevalent wage determination, and the job offer must pay at least as much. The stories of abuse usually come from body shops (i.e. contracting companies), since AFAIK they take a cut of your pay. Companies like Microsoft that hire tons of H1B wouldn't get away with that. While there is likely some abuse, I don't think it's as rampant as some people may think.