But what dictates that being cruel to entities is wrong? I think there's a fair amount of people to disagree with that, in fact there are many systems run today in which not only animals are treated with cruelty, but also humans.
Chinese labor camps, for example, are no myth. The Chinese government do not feel bad for it, and most people who buy Chinese products (which is quite hard not to do) would say "out of sight, out of mind".
Also, what is to say that we should not harm the body? I'm sure Johnny Knoxville would disagree (but then again what sort of person would be asking Johnny Knoxville for moral lessons?
) There are a lot of people modernly who are more than apathetic about their general health, smokers for example, along with alcohol users, in which neither is illegal.
You could say that there is nobody that would intentionally harm themselves though, for the sake of being unhealthy (although I could see people doing that for money or ascetic religious reasons in modern world too, I sort of do agree with Gary Jules when he said this was a mad world) but that's not the point. There are people who could look at x-ray pictures of their shot liver and shrug it off and continue getting drunk all of thanksgiving break. My claim then being; it's not necessarily better to protect yourself against harm than giving harm to yourself, another example of who might disagree are people who practice mortification of the flesh, people who self-harm (such as cutting), etc.
The fact that people would disagree means that there is reason otherwise. While I personally prefer self protection (but then again I'm not the best for health myself) rather than self harming, I would not say that self protection is the best for everyone, because some people think otherwise.