It's because the Cullens are living in Forks. He wouldn't have turned into a vampire if there weren't vampires living nearby. He isn't a werewolf in Twilight because he isn't old enough yet, but he is in New Moon.
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Jacob is a werewolf. Technically, he does not become a werewolf until the second novel, New Moon, and he remains a werewolf for the rest of the series.
Jacob in twilight isn't actually a werewolf he is a shape shifter as he doesn't posses all of the features of a werewolf because a werewolf bites people to turn them into werewolfs (their real name is children of the moon).Whereas Jacob was born into what he is but his family have stuck to just changing into werewolfes.
Jaacob tells charlie hes a werewolf in breaking dawn.
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He said that because JAcob Black was coming, and Jacob is a werewolf, enemy of the vampire ;)