Regulation NMS does provide some very important protection for retail brokerage customers, but it still allows brokers a very wide range of discretion as to how to route and to re-route customer orders. The industry norm for brokers is to take bribes from market makers, called "payment for order flow", in exchange for causing customer orders to receive inferior executions. I am glad that IB's smart router exceeds industry norms, because it intends to get the best price for the customer, rather than to maximize bribes paid by market-makers to the broker.
People who want to learn more about the broker's room for discretion should learn, among other things, about the strengths and weaknesses of Regulation NMS, and some useful info is available on the SEC's website.
