Discrimination is exactly why you want to leave that decision in the hands of HR. In the event of a lawsuit they'll have evidence to show that they followed the process your lawyers signed off upon and the decision to say "no hire" was based solely on permissible factors. At large companies, this will go as far as "here's a database dump of all the applicants with X on their criminal record... as you can see, we didn't hire any of them".
As soon as individual managers want to make those decisions for themselves, it becomes much harder to refute claims of discrimination.
That's a terrible policy. You need to optimize for getting the best employees, not minimizing discrimination complaints. How often is a felon going to sue because the company once made an exception for a specific felon because of specific outside factors, and how can that cost you any suit?
"Your honor, my client, the recently released child rapist with an iffy work record, is suing because BigCorp says they normally don't hire felons, but a few years ago BigCorp made an exception for a felon who sold marijuana, got his college degree in prison, then after he was released worked at non-profits helping the poor while getting an advanced degree in computer technology and writing several highly regarded academic papers on improving user interfaces for the disabled. That's not fair!"
The specific facts matter, and HR should be fired if they have blanket polices that ignore them. HR needs to serve the business, not the other way round.
As soon as individual managers want to make those decisions for themselves, it becomes much harder to refute claims of discrimination.