tl;dr Mars is pretty and if we terraform it then that will change its appearance. Good people don't break pretty things, and we want to be good people, so we shouldn't terraform Mars.
Nature is brutal and ugly from an ethical standpoint. Eat or be eaten. Poison, trap, munch, crunch. --And one does not generally observe moral behavior among animals, one does not observe 'selfless' acts of compassion to other species. Con-specifics can be the target of some 'selfless' acts, but in fairly limited scopes.
The only reasons we look at nature and see harmony and beauty are 1)we enjoy patterns 2) we enjoy colors 3)not many things eat us anymore.
(* New evidence may come up in future to contradict this, and that might require a change in the plans depending on the form that Martian life takes and how widespread it is)