A lot of Americans have been taught that the U.S is the land of opportunity, a place where anyone can come and through hard work can make something out of themselves. What people probably imagine when they think about this is poor immigrants coming and starting from the bottom, starting in food service jobs or doing janitorial work or something and working their way up. That's the American dream they were told about. None of them would imagine that through an H1B visa, we'd be allowing some of the most highly educated immigrants from other countries to come and immediately work in high level positions with salaries of 6 or even 7 figures.
I find it understandable that some amount of Americans feel that this is wrong and that they were told a lie.