But there are already 13 of them that are regulated. At this point BATS owns 4 of them, the only reason to have 4 different exchanges is to have slightly diffe

But there are already 13 of them that are regulated. At this point BATS owns 4 of them, the only reason to have 4 different exchanges is to have slightly different rule sets and fee structures. Any one of the existing exchanges (especially the small ones) could change their rule set or fee structure if they thought it would bring volume.