The abuse I've seen most often is that companies hire H1B workers and then contract them out to other companies, and the H1B worker can't change employers, and gets to watch most of what their time is billed for go to the original contracting company which often misleads the people about what they'll find when they arrive.

The worker ends up with very few choices, and the companies at which the workers are working are likely overpaying for what they're getting. The only one truly happy is the "placement" company that holds the workers' contracts.