The aesthetic issue
Let's not beat about the bush, it's a basic observation: we don't like other people's poo! It is all the more taboo in our modern societies with their shiny (or even connected) toilets.
So when the piles of toilet paper overrun the trails, on which we have come to find peace and quiet, it is like a cold shower!
While this problem is not often addressed here, it is more common in the United States, for example, where the big parks suffer from overcrowding. The major problem arises because of the high concentration of people on a particular site.