I can't agree with letting this (little) thing go because there happen to be atrocities in the world. NYC got its subway car cleaned up, for example, by not tolerating even the smallest bit of graffiti. I would not be confrontational, but ask politely that the poop be picked up for the reason that I walk back there and might step in it, particularly at night. It's very likely that the offender was carrying no plastic bag or would otherwise have put it to use, so I would offer to provide one. If the offender becomes angry or insulting, then so what? Nothing has been lost, because that would not be a person you would want to have further dealings with.