Are. And since you made the issue personal, I feel the need to respond and clarify the purpose behind the whole B.A.D.D movement, if you will.
Dragons are, indeed, there to be killed. They are monsters designed to challenge a group of PCs, but ultimately they are defeatable. I've used dragons in my campaigns three times over the course of 25 years of DMing. The PCs were able to defeat the dragons all three times, and there were no TPKs.
HOWEVER
Dragons have a lot of abilities, physical and magical and social, and are quite powerful. In order to provide the maximum amount of challenge, DMs should be familiar with these abilities and make sure to take into account the kind of tactics a generally highly intelligent beast with hundreds of years of experience would use.
This kind of preparation would be needed for any type of similarly powerful beast.
Dragons are iconic. An encounter with them should be something adventurers (and their players) never forget. I have seen an aweful lot of forgettable encounters, though, and this is what made me Bothered.
My philosophy about dragons springs from the works of Tolkien, especially how he portrays Glaurung in the Silmarilion (and reading Children of Hurin recently only cemented this belief). Glaurung is able to manipulate and destroy many lives. Ultimately he was killed, but he left behind a lot of pain and misery. He wasn't Disposable, and he remains quite memorable.
By the way, I'm sorry for being so obnoxious.