For this reason alone, Betfair has no reason to ban or close your account when you should win. This is what Betfair first became famous for; peer-to-peer betting. Whilst the Exchange account won't be closed for winning, there is an additional commission rate.
Therefore it is Betfair that set the prices and carry the risk. The odds are generally higher for Singles bets (Exchange) than for the same selections in Sportsbook, but unlike the Exchange, when you place Sportsbook bets you will not pay commission on your winnings.