I don't know why Hertz is making a big deal out of bringing the cars back charged, other than as an excuse to make some extra cash. The cars probably sit there long enough that they could plug all their Teslas into 120V outlets and charge them fully; their only expense would be a little bit higher electric bill.

Now whether they should charge them fully is another story. No higher than 80% is best for long battery life, unless they know the customer is picking up the car today, in which case 100% is fine.