That is most likely because people have an ethnocentric point of view.
In the first case, let us imagine one culture has the belief that they should kill any rabbits that might cross their path. This culture does not have an infestation of rabbits and the rabbits are not affecting their crops. The people of this culture just believe it to be morally right to annihilate any rabbits that they see. The mere act of killing these animals makes the killer feel great about who the person is. As Americans, some of who own and raise rabbits as pets and loved family members, for the most part would find this appalling and morally wrong. Does that necessarily mean that killing rabbits in said culture is wrong? Can we, as Americans, blame the people in this culture for killing rabbits? If you are a cultural relativist, then you cannot blame or condemn the people in this culture and you have to believe that it is morally right for them to do this. The same is true if you are a moral relativist. The people killing the rabbits do so because of their own system of beliefs. Therefore, if we agree with moral relativity it follows that killing rabbits is perfectly natural for those people. .
An example of moral relativity without a cultural basis would be a person killing another person because they feel like it. The killer has no remorse and does not necessarily think killing that person is the morally right thing to do, however the killer does not believe it to be at all morally wrong either. The killer just kills people because he has no history of moral upbringing on the matter of murder and has had no reason to believe that what he is doing is wrong. Can one say with their own belief system that he is wrong? If one believes in the lessons of moral relativity then one would not be able to condemn the killer. They would have to say something like "It takes all kinds." One would have to accept him for who he is and what he does (moral baggage and all).