Someone forgot to tell the Tucks that the truth will set you free. And so, in Tuck Everlasting, they're stuck, never able to admit to the rest of the world who they really are. They're living a lie—through and through—but can we blame them? (Really, we're asking.) After all, they're lying to protect other people. The entire world, actually. Who knows what kind of craziness would go down if everyone knew about the possibility for immortality? But all that doesn't change that they're lying—and asking Winnie to do the same.
The lies that the man in the yellow suit tells are clearly the bad kind. The Tucks, on the other hand, are just lying to protect people.
By the end of the book, Winnie's been totally corrupted by the Tucks: she's willing to lie to her parents and the constable—break the law, even—in order to help Mae Tuck go free.