To me, it's a matter of creating liberty and justice for both the employer and the employee in the matter, with neither being slighted.
If an employee is terminated without being provided an accurate reason, is either the relevant freedom or security of either the employee or the employer thrown (more) toward imbalance?
That is the question that I would seek to answer.
Without engaging in a more deeper analysis at this point, it appears that the employee is having his security slighted at the benefit of the employer by not receiving a real reason for his termination. Thus the employee is not receiving justice.
Thus if the employee is really being laid off, but is not told that, he won't file for unemployment if he thinks he's being fired (being fired doesn't entitle one to unemployment benefits), and thus the employer can just let it go as a "firing" and not have to pay additional unemployment premiums.
So the employer's freedom, his liberty, is unjustly increased at the expense of the employee's security, his justice, if he does not receive a real reason as to why he's being terminated, whether it's indeed just a real firing (as the thread title suggests) or a layoff disguised as a firing.
Thus, in the interest of liberty and justice for all, I would say, at this point, that an employer should indeed be legally required to provide (have) a real reason to (give to) the employee.