Believe it is due to RegNMS Order Protection Rule 6.11 : Change should reduce Trade through violations exchanges are responsible to police and enforce. These outliers cause regulatory violation for the exchanges ($5k to $200k fine per violation albeit rarely enforced). Brokers are on the hook for failing to provide best execution - Common law breach of Fiduciary Duty claim - Most investors are not going to file FINRA arbitrations... Probably substantially more will in this distinct class of outliers. This puts both the broker and the exchange at risk.