$100 T-mobile Galaxy s3 w/contract at Walmart + $200 etf = $300 Galaxy s3?

[finkalicious]

So basically if I were to stay on contract for a month and cancel it, t-mobile would only charge me $200 for etf, essentially making the phone $300 total cost? Maybe $400 after activation fees etc.? I would stay with t-mobile afterward, just on a prepaid or value plan, but I'm trying to figure out what the drawback for doing this would be...this can't be possible right?