As other noders have already noted, it's quite true that methadone is quite a bit more addictive than heroin. I've heard junkies say that "You can quit a smack habit, but methadone is forever." It's also true that methadone is at least as bad, if not worse, for you, than heroin, though the fact that methadone is administered in a clinical setting reduces many of these risks by quite a bit.
But the question remains: Why methadone maintenance therapy instead of heroin maintenance therapy? The reason is simple: methadone doesn't produce euphoria. The War on Drugs rhetoric, is I think responsible for this neo-Puritanism, the idea that anything that makes you happy must be a sin. More proof to my mind that the War on Drugs' basic impetus isn't reason or anything related, but just as fearful of pleasure as religion.