If one dog leaves one dog poo then no-one will ever notice.
But if you have an area like Snowdonia, which is visited by several million people each year, then it’s a different story.
It’s a fragile upland environment, with rare plants, animals, and even fungae and bacteria. Upset that balance with a few million doggie dooes each year and now you’re looking at significant damage to the biodiversity that everyone goes to visit in the first place.
Same is also true of human faeces.
And that’s before you start worrying about what happens when it gets washed into streams that people and animals further down the mountainside might be drinking from.