Of course, non-human animals do not generally have the legal right to not be killed for frivolous reasons. However, you should consider what motivation would be strong enough for you to kill a cow yourself. I know that some people would answer yes to this question. Don't worry about those people. Think about your own answer.
Nature is cruel.
That is true. However, wolves wouldn't kill cows in order to get covers for the Palm V handhelds. Also, we are capable of making moral choices which wolves are almost certainly incapable of making, and it is unreasonable for us to blame wolves for not doing something they are incapable of doing. Actually, I often do just say animals. Here I'm stressing the point that humans are animals too.