Our entire world couldn't be more far away from what nature once was. And there are just so many "artificial" things that have proven to be better than nature. What I'm saying is that just saying that something is natural really does mean nothing. I can be both, good or bad. It's not like I'm denying nature. Because you said that I need to be aware of it.

Why are some things that some animals do used as a justification for humans to do the same, while other animals doing something else isn't?

For example, Wikipedia says this about the topic:

Territoriality is only shown by a minority of species. More commonly, an individual or a group of animals occupies an area that it habitually uses but does not necessarily defend; this is called its home range. The home ranges of different groups of animals often overlap, and in these overlap areas the groups tend to avoid each other rather than seeking to confront and expel each other.

https://en.wikipedia.org/wiki/Territory_(animal)

It is natural to not have borders, and only a few species do.