That's not my experience with it. The people who are typically given the waiver are tech and science workers, and their salaries are in line with what anyone else would be paid in those positions. It functions more like a stopgap to fill positions they're unable to fill with the current domestic labor pool than as some money saving measure. But I'm sure there are cases that reflect what you're saying. I'm just speaking on my own firsthand experience.
Discussion
It is also super hard to know if you are being underpaid or not, a lot of US workers get underpaid too.
It can also manifest in the H1B workers not getting pay raises or extra discretionary pay. When it comes time to raise salaries companies will almost do it on the basis of whether or not that person will leave to a competitor. The H1B employees have a much harder time leaving to a competitor.
Big companies will even collude to stop their employees leaving between each other, so that they can keep all of their wages now and avoid competition on salaries.
I've worked for two companies that used H1B visa workers, and I knew what they were being paid. Their salaries were in line with domestic workers in their positions. Now, my experience is based on a small sample size, but in talking with those employees about the H1B and their related experiences, I didn't come away with the feeling that these employees were being underpaid. But again, that's just based on my personal experience and what I've been told by employees in that program.