that's pretty much why i dont enter any contests online, they all seem dishonest to me
https://twitter.com/CarnabyLondon/status/220192719958581248
But then again, it's not some unknown startup, it's a local marketing initiative and I can meet the organizers face to face just around the corner.
If your time is so valuable that taking the time to (re)tweet something will be a drag, or if your twitter account has some intrinsic worth that could be brought down by this type of tweet, then yes, there's no reason to enter, but then again, if you meet either of these criteria, you probably don't even need to enter contests...
Edit: full disclosure... I frequently tweet food pics and almost never tweet content entries. :)