IEX is trying something slightly different to get around allowing/paying market makers into their dark pool. They are using publicity and Michael Lewis to try to convince retail investors to provide free liquidity for their backers.
I guess "for free" was a bad phrase. I meant that IEX is hoping for retail investors to make up for the lack of market makers who get paid via the bid/ask spread.
It remains to be seen if this tactic will work.