American citizens are granted many rights in this great country we live in.
I'll make this simple.
There is a minimum wage that employers are required to grant to their employees that are legal citizens of the United States.
To hire an illegal immigrant is to be able to hire somebody for very cheap, less than minimum wage.
If they are made citizens (to some extent), they will be required to earn minimum wage.
Here's the interesting part.
While employers options slowly become smaller, more American citizens will be able to snag jobs from those who are less required.
Even if not, the country will still have a taxpaying citizen in the workforce.