I don't see why you think that at all. Re-tweeting, sure, but that's because it's an expected result of putting something on twitter, with or without tags or even text.
Sending someone a picture on twitter in no way implies I am giving it to them to do whatever they want, in the absence of external context.
Or in other words: The tweet as a whole is released to the public of twitter. There's no license to derive or take elsewhere.
Sending it directly in the mail doesn't let them use it to advertise either.