As a dog owner I know that the vast majority of dogs' mess is cleaned up at the time by the dog's owner & if you need proof try opening one of the red bins in the park on a hot, sunny day!
In reality if a dog owner leaves his **** behind then the other dog owners are very vocal in telling them to go back and pick it up. Obviously there are some who are inconsiderate but their actions are just as likely to end up on the bottom on my shoe as it is on yours.
However if you're keen on taking your turd-hunt up a notch can you have a word with the foxes at the same time. They come over here, rumaging in our bins, eating our left-over food then go out into the streets and crap everywhere. They never pick up after themselves & it always looks suspiciously like dog ****