I'm sorry that you had that experience; I applaud you for trying to do something about it.
Unfortunately, there are any number of people out there who simply do not know the importance of basic safety rules like using an appropriate, non-expired, non-crashed child restraint. It is even more unfortunate when some of those people hold positions in which they're making decisions about child restraints. Knowing now what you do, I'm sure that you realize that it is risky to put your child's safety in someone else's hands. As a parent, we want to think that everyone has our child's safety in mind and can be trusted to make safe decisions, especially people who really ought to know better like car rental employees, but as you found out, that is not always the case!
Let us know if anything comes of it. If you're interested in pursuing things further, you might investigate state laws in the state where you rented the car and see what regulations there are regarding child restraint use, and if Hertz was in violation of any of them. Be sure to check specifically for laws related to car rentals, as they might be different than personal passenger vehicle laws.