This is highly illegal and taken very seriously. Is this speculation on your part or do you know this for a fact (in which case, you should report it).
It's been widely reported [0]. And as I said I feel like it's still a fair deal. Some HFT's get my orders and they know I purchased 2 AMZN yesterday a few seconds before it executed. In return I get to trade for free on their platform.
They're probably not front-running. They are almost certainly selling trade information to HFT firms.