Right next to the "Submit" button, it says that the tweet will be sent out. Also, the button itself says "Save and Tweet Your March Madness Bracket"

I still get spam from colleagues who signed into Quichup years ago and gave it access to their gmail accounts.

This behaviour is absolutely Evil. Why do companies think that we want such coup d'etats in our accounts?

To be fair, it does clearly say that you must send out a tweet to be eligible. I didn't go through the process so I don't know if it made it clear that it would do that for you.