Unlike traditional bookies' Starting Price, there is no margin for profit built into the Betfair Starting Price so it often gives you better odds. Commission is the amount you are charged by Betfair on your net winnings on an Exchange market.
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.