"At Will"?

I've recently discovered that Illinois is an "At Will" state. In a nutshell, that means that any employer can fire any employee and not give any reason...any reason at all. Apparently, the only thing that a fired employee can do after being refused an explanation for the termination is to request (in writing) to see a copy of his or her personnel file. Then, hopefully, the person can get a sense of "why" based on what's in there. If the employer refuses to present the file, then the Department of Labor can be contacted and said Dept will tell the company to produce the file. After the file has been presented, they have fulfilled all of their obligations to the ex-employee.

That's insane! What idiot put that law into effect??? What is wrong with a business being expected to present a reason for employee termination?

Admittedly, I understand that most businesses do indeed present reasons for terminations even though they are not required by law. That makes me even more suspicious of those who do not. What are they trying to hide?