Actually, I can't understand it. What was wrong for "big business" with the status quo. (assuming arguendo that "big business" is hiring these folks) If illegals are paid less than legals for similar work, and the bill eliminates the illegality, it would seem to me that the number of underpaid workers will decline drastically.
ML/NJ
If you are running a big enterprise, you cannot now use illegals to directly replace (and compete with) the relatively highly paid citizens in higher positions because you need reliability. Illegals could be arrested at any time besides opening the organization to legal risks and liabilities.
The boomers are all retiring - there are fewer workers and more wage pressure diluting profits. Bring in more competition and your worker drones are forced to compete much more desperately.
Mass layoffs are only partially successful because they are so disruptive. Why not do it more subtly and incrementally?