If IB front runs you, call the SEC. That's highly illegal. It's far more likely that they just use your trades to fill their orders. That's a good thing for both you and them.

The mechanics:

IB wants to buy X at $100, you want to sell $X at 100. Normally your order would be routed to the exchange and so would theirs. Both would go to the back of the queue: if Foo/Bar placed buy/sell orders at $100 before you, they get filled first.

Instead, both you and IB get to bypass the queue by trading directly with you. The only real cost to you is that this probably incurs some latency.