What if the posting is not made at the end-employer, but at a contract firm who supplies temporary laborers to the end-employer. In some cases, the "firm" is a PC in the family room of some guy in the hills of Fremont, CA.
There are requirements that the employer has to demonstrate (i.e. they really do exists and they have the ability to pay the wage). Annual reports are typical as well as supplying the empoyer's tax ID. There are 'living room' shops, but a hired emplee on H1B has to work for the sponsoring employer, and not be contracted out. That's where the big abuses come from.