And these exchanges have historically been doing it. BATS has been around for about 10 years. But the initial players like Island ECN, Archipelago, and others (now merged into NSQD, NYSE, etc.) have been paying retail brokers for their flow since late 90s. This is what attracts big fishes to trade at their venues.
When you do the wrong over several times without getting caught it becomes a standard (read: make-or-take rebates)
