Bean-counters take a bit longer to understand this (in my experience).
I'm a bean-counter and I totally understand this! Nearly all of my bills that go out to my top clients have some services performed for them with "No Charge" in the fee column. Could I charge them for them? Definitely. Would they be upset if I did? Almost definitely not. Then why do it? Because people like getting something for nothing and keeping these guys happy means they aren't looking at other service providers so the big ticket items bring in the fees for me.
There is an interesting book that I've read recently by an American car dealer (Sewell I think he is called but I could be wrong). His view is that it is better to be ripped off occasionally by a chancer than to piss off a loyal customer. And I agree with this entirely! It costs a hell of a lot of money to get a loyal customer so you don't want to lose them to a competitor without very good reason.