New Hyundai tiburon, cheated out of warranty, why?

I have a 2006 Hyundai Tiburon w/ 35K miles on it. Last year at around 17K miles I took my Hyundai to a local dealer in So.Cal for possible car defects. Everything was ok, fell under waranty, $ owed= $0. This year I went to a Nor.Cal. Hyundai dealer b/c I was having problems with my sunroof. It turns out it was defective. So I got a bill for a diagnostic fee. At the time they did not see anything wrong w/ my car. But one of the service attendants decided to check it out again and they found a problem. So why did I have to pay a diagnostic fee?

I called Hyundai and they said that the service falls under that warranty but a Hyundai authorized dealer is not required to honor it. Is this legal? What can I do? I feel so cheated that I bought this car specifically b/c of the warranty and to avoid fees such as this one and it turns out that the warranty is optional? Is this legal?