The issue of constitutionality in this particular case doesn't really have much to do with protected classes. And no, it doesn't take hours to explain the reasoning as to why portions of this law may be unconstitutional.
First, it should be noted that this law includes a severability provision, stating that “If any part of this act is declared invalid or unconstitutional, that declaration shall not affect the part which remains.” So if the only section found to be unconstitutional is the one which prevents employers from claiming wages paid to illegals as deductible, then all other sections can still be validly enforced.
There are a variety of sections that have been challenged for a variety of reasons; it's not as simple as stating that the law as a whole has been challenged for only one constitutional reason, such as the underlying effects.
The majority of the claims as to constitutionality are based upon federal preemption. As an example, Section 13 of the law creates an Alabama-specific harboring scheme that “removes any federal discretion and impermissibly places the entire operation – from arrest to incarceration – squarely in the State’s purview.” (Cited from
Judge Blackburn's memorandum opinion entered on September 28).
The opinion also found that sections 11(a), 13, 16, and 17 would likely be deemed unconstitutional, based upon the fact that they preempt the federal government by placing immigration operations within the State's purview.
I think people know that this is a law against illegal immigrants, and I think they immediately jump to conclusions about protected classes when the constitutionality of such a law is brought into question. But, in actuality, the majority of the claims which are being considered as having merit are based upon federal preemption.