Hi Ian, and welcome to the board!
Although 'can animals sin?' is a perfectly valid question and could start an interesting discussion in another thread here, the reason I used the 'why' theme was to show that humans are unique in this way -- a way having nothing to do with theology. The human ability and evident need to ask 'why' about all manner of things not even relating to daily survival appears to be unique in the physical creation. Not just unique, but unique as separated by a wide gulf between the rest of physical creation and human beings.
From babyhood children are curious and the "Why" questions that start erupting at about 2 1/2 or 3 are famous for driving parents a little bonkers. As a mother of six (now grown) I can testify to this. This driving curiousity regarding the PURPOSE of things never shows up in animals. Even "how" is not really apparent even in the simians or dolphins, let alone dogs, cats, or horses. They don't care how something works. They only care IF it works and IF it results in something good or bad where they are concerned.
Again, there is a giant gulf between humans and animals that the evolutionist blinds himself to but which the other human beings of the world cannot help but notice.
Now, I would also say that animals are most definitely NOT machines God made for us to use. They are not machines, period. They, according to Genesis, have nephesh, the breath of life, also translated as 'soul' and 'heart' (meaning character or personality). They are precious parts of creation that we are to care for. Some serve us; most don't. Nephesh animals appear to be those which have complex nervous systems -- at least that is how I, personally, have connected the created kind with the concept of 'nephesh'. It does not seem to related to insects or spiders or worms, or bacteria, etc., at all.[/b]