Discounters that charge deeply discounted commissions either make markets, sell their order flow, or both. These sources of revenue enable the cheap commission rates as they profit handsomely from trading with your order or selling it to another. Market making is the answer. In contrast, Datek is one of a kind. Datek charges a fee from Island subscribers to enter orders into their system.
Island is their outside revenue, and is far superior to selling order flow.
Island is good for the customer, selling order flow like the others is not.