As part of the H1 filing process, employers are required to post an LCA (Labor Condition Application) at the workplace. This is an official-looking document typically stuck on a pinboard around the workarea. Amongst other things, this explicitly lists the prevailing wage/salary at which the H1 is hired. So, in effect there is nothing hidden from other employees or the employee being hired. I have second hand knowledge that LCA placement is in fact audited. In any case, LCA placement has been enforced by the legal department in my last couple of workplaces.
>About half of all H1B holders come from India alone.
Yes, that is true and I'm not denying that. My point is to clarify misconceptions about the H1 Visa program itself being legal slave labor and focusing on highlighting the subtleties that give it this perception.