I too have seen a lot of irrisponsible dog owners here in the UK
When I was younger, my parents got a dog and we were always taught to be responsible about it if we took him for walks, as in make sure to always take a plastic bag and clean up after him. So it kind of makes me judge people who are a lot older and I see them just walk away from it.
I walk into town almost every day and there is never a day that I don't see flattened dog poo on the cement. It gets rather irritating because I feel like I am weaving in and out to avoid it and it's just disgusting. I've never seen anything like it in the US. It's just disrespectful. I walk through a park on my way to town and there are bins for dog poo, so it's not that difficult people! I find it funny that it says "£500 fine for dog fouling" (or something like that) but EVERYONE does it and obviously there is no way to monitor it. I have never seen these black bags that you speak of, because it seems like here people either make the full effort or no effort at all.
I am also irritated by dogs without leashes. Like I said, I had a dog myself, so I am not afraid of dogs coming up to me. I know *most* dogs wouldn't just run up to someone and bite them. Though there is a lot of cases in the US where dogs that weren't on leashes attacked children and in some cases killed children or other dogs. Also whenever I see a dog without a leash it confuses me a bit because if I saw that in the US, I would assume the dog had run away from its owner and was lost.
One more issue I have is that people where I live seem to let their yappy dogs sit in their yard all day while they are gone. It irritates me so much because when we open the windows it's all we can hear. The dogs bark for hours on end.
I guess its just another one of those things that comes with living in a different country and adapting to the way people do things here. The laws obviously aren't as strict here.