"And I can throw this one back at you.. what incentive does a private roadway have to thwart illegal activity that does not impede its operation? What keeps highways from being free passage for all sorts of black market activity?"

I'm not trying to be petty but there is no such thing as a black market in a totally free market. If your referring, to weapons, drugs, and sex trades, I'll give you one reason: Who wants to drive on that road? Is there anyone who'd prefer a scary road to a nice one? Unless these people are driven by something else besides profit, they would certainly keep the road "clean."