It’s great that you have created a twitter-enabled app for showing off your newest product line or allowing your customers to enter sweepstakes to win some great stuff. I’m cool with that.
But never, never tweet on user’s behalf (using access to their twitter account they’ve given you by logging in to the app) without them knowing that & without them giving you distinct permission for that EACH TIME.
This can be compared to shopping mall putting stickers with their message on their customer’s cars while they shop. Just like anyone’s car is their personal belonging, anyone’s twitter feed is property, which says something about the person to the world. By not letting your customers choose what to say and invading their territory, you show your disrespect towards them. That’s a dealbreaker for any future relationship this person could have with you.