In my mind, it's a matter of inertia - if "protected" is default, then if an employer wishes to have the ability to fire anyone at will, they need to offer concessions - whether it be a higher salary, or the ability to quit at will, or whatever.

However, if "not protected" is the default case, then it becomes far more difficult for an employee to obtain such protection, as it's not beneficial to the employer. And as such, barring anyone who can negotiate their own contract (the minority, generally), most people will remain unprotected. And I believe that to be a problem, for reasons I mentioned elsewhere in this comment thread.