The intellectual justification is probably something like: Hiring someone always represents a risk. You don't know if the person is going to succeed at the job

The intellectual justification is probably something like: Hiring someone always represents a risk. You don't know if the person is going to succeed at the job you hired them for. You don't know if they're going to wind up a positive asset to the company. You need the option of firing them, and the possibility that you might wind up in a legal battle because you did so -- that even if you have an airtight case, you'll have to lawyer up and spend time in a court room -- is scary and could therefore have a chilling effect on hiring in the first place.