If you see this happening, then you can actually report it to DoL (I have had a call from a DoL investigator about H1B abuse at a past employer based on a complaint filed by an employee).
I've worked at several big tech companies and they all abused H1B's. The immigrant workers were paid a lot less than citizens. It pissed me off because the immigrant workers were treated like crap and also I would imagine it puts downward pressure on my own salary.
I worked at a company that gave referral bonuses to employees for new hires. It went like this: $2,000 for a white. $3,000 for a black or Asian. $5,000 for an Indian. HMMM.
There's little incentive for an individual to report anything. You've seen how "whistleblowers" get treated. Up against a big company with a legal department, chances are good that they know how to put up chaff, redirect, deny and obfuscate their abusive practices. Why stick your neck out when it's a systemic problem?
So you are correct that they have to be paid market wage. But the details matter here. For the DOL, the "market wage" is the "prevailing wage". This is a function of the job classification (title), wage level, location and (most important) which wage survey they choose to use.
By now you can probably guess how a company that wants to underpay can use these levers - under classifying, moving them to a suburb with less employers for that job, using a different wage survey etc.