The point is that the payroll tax results in an employee taking home 15% less money than he otherwise could were it not for the tax.
The other way to see it is that the employer pays out 15% than he knows the employee will accept for his work (i.e. his net pay). If the employer suddenly doesn't have to pay this, it's gravy, and you're assuming that he'll see it in his best interest to pass that gravy on to the employee rather than apply it to his bottom line and benefit his shareholders. I think, based on my interactions with the real world, that this is delusional.