I guess that this is, to some extent, a cultural thing.

In Europe, it is customary to round up a taxi fare, to tip in restaurants (though nothing like the 15 - 20% that seems to be expected in the US) if service charges have not been added to the bill and that's about it. Americans seem to think that just anyone can and should be tipped just for doing their job. I suspect that this means that Europeans are seen as plain mean when it never even occurs to them to do so, and conversely in Europe, it can actually cause offence in some situations.

I was under the impression that it is Hertz policy, officially, that employees are not supposed to accept tips (or bribes), though I have no doubt it goes on. I have on more than one occasion take then time to write an email to the location manager or regional manager to praise someone by name for excellent in customer service, and I do know that these compliments do find their way to the staff and they appreciate them.