Unfortunately this is true. Most people [in the US] don't tip on the amount of a coupon and sometimes even gift cards.
I'm sure when waiter's see my wife and I with three little ones come in their heart sinks a little thinking they're going to get a bad tip. Especially when we split a plate for ourselves and/or our children. However, when the bill comes I tip around 25% of the bill and then add approx. $5 per split dish [when the restaurant doesn't already have a charge for doing such] when means they come out with a very nice tip percentage wise. ($5 is often less than a dish so I still save money, and a $5 tip would be 20% of $20 so the waiter is still tipped for putting up with my family and bringing out an extra plate.)
I'm sure when waiter's see my wife and I with three little ones come in their heart sinks a little thinking they're going to get a bad tip. Especially when we split a plate for ourselves and/or our children. However, when the bill comes I tip around 25% of the bill and then add approx. $5 per split dish [when the restaurant doesn't already have a charge for doing such] when means they come out with a very nice tip percentage wise. ($5 is often less than a dish so I still save money, and a $5 tip would be 20% of $20 so the waiter is still tipped for putting up with my family and bringing out an extra plate.)