Isn't that both sides though? It automatically fills you at the next price whatever it is. I don't mind that. If it trades through my price then I should get filled at the next price. Sometimes it's in your favour and sometimes against, but at least it's symmetrical. What are you calling slippages? 0.5 ticks etc..?
Things like the virtual dealer plugin are designed to ensure it isn't both sides, or at least not equally. FXCM made millions doing it, it was free money for them and god knows how long they got away with it. I would guess that all brokers are at it to some degree or other, perhaps using different tools.