Why isn't the inverse true? When you have a call back (rework) do you reimburse your employer for labor, travel and parts? In that case you are charging your employer but he is not charging the customer. Why isn't that considered a "ripoff"?
How about the automotive industry. When a tech is on flat rate and there is no work they have to stand around and wait for a customer to come in without getting paid.
If we were in a high profit industry where upper management and ownership make upteen times more than you do I might be a little more sympathetic.
Your employment relationship is a 2 way street. You give, they give. Unfortunately for you, you don't own the company so typically if you don't give first then you probably won't receive as much in return as the other people that do.