Well, see the TAB have an association with some other company who actually provide the cash payouts. TAB are just an intermediary in this case. The other company is basically providing insurance, and to do that at an acceptable risk to themselves, they will effectively place wagers on all the outcomes that you did not select. The transaction costs involved in doing that (i.e. copping the bookies margin on each bet) add up.
They still rip you off absolutely, but there are good reasons why the cash payout will be lower than the simple single event odds.