I did shout to two shaven headed guys who had 4 staffies (or were they stuffy x pit bulls, very large dogs anyway) - hey, I think you missed this, one of your dogs just crapped in the middle of the path. They turned and shouted back, what am I supposed to do about it. I said "pick it up and put it in the plastic bag I'm sure you have in your pocket". The owner instantly pulled out his pockets, no plastic bag. I said in "don't worry, I've got spares" and waved one at them. I had 3 dogs with me, one of which is just a bit reactive and fearful of dogs bigger than him, I felt similarly about the men. Their response - to hurl an earful of abuse at me, after which they swaggered off to continue the 3 mile walk around our lovely reservoir.
Most folks are delighted if you call out that they missed their dog and offer them a pooh bag, but there are always one or two nasty folks around.