Now, if I own a business, it's mine.
How would you feel if you put 40 years into "your business" and after all that time someone said they would like to buy it for 20 million dollars, but before you sign the papers, the state you live in, claims eminent domain on "you business". Would you still feel that way?
But we are not talking about a negative factors here, but positive factors.
If you need to down size the company, then you have a right to downsize the company, a negative factor.
But if you are firing the person for a positive factor, ie you have to pay pension, you want to get even with him for sleeping with your daughter, or anything else that like that, it should be illegal.
At-will workers have no contract and therefor no legal basis to fall back on. By forcing both parties to sign a contract, you can make the working environment fair. It does not mean you can not legally position them into a at will state, but it means they have to openly agree to it. If you state that you would give them pension, fire them only to protect the company, and give them a christmas bonus, then they have a contract to legal back up.
Laws against at-will employment does nothing to harm your business, but requires you to set the ground rules in writing before you hire anyone. And if you refused to have something in writing, the law will give you a default set of rules. If you don't like those rules, then make sure you get them to sign the your contract.