While I agree with the sentiments completely, I would caution the more polite approach to dealing with folks - simply because you don't know the entire picture.
As an example, during my last camping trip, I had tossed a bunch of baggies into my vest pocket and headed out with my two dogs for a morning hike. Picked up after them with no problem. After lunch, I again took the dogs out for a hike. We walked from my campsite to the head of the trail, got about 5 yards down the trail and realized "****e! All the baggies are in the vest in the RV". We turned around headed right back to the RV. Before we made it, both dogs had to relieve themselves. I practically ran the rest of the way to the RV, grabbed the bags, raced back to the offending piles, and cleaned up, then resumed our hike.
In my situation, I had no intention of leaving the piles, I was not an irresponsible pet owner (just a forgetful one). Had someone passed by and offered me a bag at the time of the deposit, I would have been absolutely thrilled. As it was, I was expecting someone to start accusing me of being an irresponsible pet owner as I "raced away" from the scene of the crime.
So, don't assume that every person who walks away is being irresponsible, they may have just been unprepared. Offer them the baggies, but do so politely and without accusation. You may find your efforts are much more appreciated by all and you may actually turn some of those irresponsible pet owners.