Why employers must hire DREAMers rather than citizens and permanent residents?

There have been many instances in which employers have been reluctant to hire DACA approved DREAMers and the reasons are understandable.  These aliens have no permanent right to work in the United States so if you invest in training them, it might be wasted.  If the job requires overseas travel, it is a hassle to have a DREAMer employee.  However, it turns out that the best reason to replace a US citizen or permanent resident by a deferred action DAPA or DACA individual is cost savings by not having to pay insurance premiums under ObamaCare.  Under current provisions of the Affordable Care Act (ACA) these folks are not eligible so as an employer you do not have to pay a penny for them and they are on their own to find healthcare.  According to many estimates, the savings can be thousands of dollars.