There are plenty of cases in which the company just needs to have a policy that it's an unacceptable risk. For instance, if the job involves getting a security clearance, a felony on your background check basically means that you are unfit for the job. Or if you are dealing with highly sensitive financial data, there can just be too much of a risk.

I'm not saying that all HR departments should have such a policy (in fact, in some cases it can be illegal to request criminal background information, at least on the initial application form). But there are some cases in which a policy like that could be necessary and final.