The only information they have is about trades that are about to hit their private trading network (before they go out to the market as a whole).
It would seem to me that if this were a problem, private networks would emerge that either disallowed the look-ahead option or allowed the party placing an order to declare it off limits to look-ahead (which would probably result in an increased transaction fee).
So in other words, the only time this could really result in exploitation is if one party lacked the wherewithal to use a different trading network (or a large trade broker) when trying to do a large transaction a few shares at a time.
This seems like the sort of thing that would attract innovation and would be fixed before anyone could blink, as trading network firms realized they could make more money by offering both options. My guess is that the lowered fees available in the sort of ecosystem that exists today would limit the demand for a trading network that disallowed it. After all the feature was added on as a desirable product fairly recently and nobody complained about it... until HFT became the scapegoat du jour for the financial crisis.
