I am one of those who picks up after her dog. That out of the way, I wish to observe that Mother Nature and the Pacific Ocean are much more accepting of poop than of plastic (including butts and caps and all the rest of it.)
This is NOT at all to suggest that we leave our dogs' poop; I do, however, suggest that the real damage to the environment has nothing to do with what our dogs leave behind.
We would all be better served, of course, if each of us took responsibility for our own crap.