Yes but for proprietary traders using their own capital, it doesn't make a lot of sense to force them to trade in a particular way since executing sub-optimally can only impact their own performance. There are legitimate reasons not to trade the lowest price market (maybe you believe the quote is slow, want to trade bigger size all at once, don't want to risk signaling to the market): http://www.hudson-trading.com/static/files/RegNMShudsonriver...

And retail brokers already do what you suggest. All their marketable flow gets sold to off-exchange market makers, while their limit order flow gets routed to exchanges that pay the highest liquidity rebates, Reg NMS just mandates what price it can trade at: http://news.indiana.edu/releases/iu/2014/02/study-of-potenti...