The bet was made January of last year, waaay before the draft took place. It's easy to say he had no chance to win the bet now, with the benefit of hindsight. But back then, there were a lot of people convinced Miller was going to completely bust like Maybin. If Miller had a rookie season like Maybin (0 sacks), I would've owed Thoner money and I would've paid.
Plus, who cares? Even if it was the most lopsided bet in the world, a bet's a bet and there are no takebacks. Do you think Vegas is refunding Indy-wins-the-Super-Bowl bets placed last April?