They're obligated by the regulators to ensure best execution, which typically means they have to do statistical analysis of the execution quality. Routing an o

They're obligated by the regulators to ensure best execution, which typically means they have to do statistical analysis of the execution quality. Routing an order away doesn't absolve them of this requirement. However, they have a business incentive to optimize for the greatest kickback. So it's a question of ethics. And those are sometimes in short supply in the financial industry.