It's not a difficult thing to understand, 5 minutes on Google would help, but I've done some of the leg work for you to save the bother.
The TLDR answer to "why is this not self harm" etc, is "intention".
Not understanding something is no reason to disregard it as "an abomination to the human race".
Self-harm or self-injury is intentional behaviour that is considered harmful to oneself. This is most commonly regarded as direct injury of one's own skin tissues usually without a suicidal intention. Other terms such as cutting and self-mutilation have been used for any self-harming behavior regardless of suicidal intent. The most common form of self-harm is using a sharp object to cut the skin. Other forms include scratching, hitting, or burning body parts. While earlier usage included interfering with wound healing, excessive skin-picking, hair-pulling, and the ingestion of toxins, current usage distinguishes these behaviors from self-harm. Likewise, tissue damage from drug abuse or eating disorders is not considered self-harm because it is ordinarily an unintended side-effect.
Body modification (or body alteration) is the deliberate altering of the human anatomy or human physical appearance. It is often done for aesthetics, sexual enhancement, rites of passage, religious beliefs, to display group membership or affiliation, in remembrance of lived experience, traditional symbolism such as axis mundi and mythology, to create body art, for shock value, and as self-expression, among other reasons. In its broadest definition it includes skin tattooing, socially acceptable decoration (e.g., common ear piercing in many societies), and religious rites of passage (e.g., circumcision in a number of cultures), as well as the modern primitive movement.
In many ways self-mutilation is very different than body-modification. Body modification gives one the feeling of pride and excitement, giving one something to show off to others.[24] Alternately, those who self-mutilate typically are ashamed of what they've done and want to hide any evidence of harm. Body modification is explored for adornment, self-expression, and an array of many other positive reasons, while self-mutilation is inflicted because of mental or emotional stress and the inability to cope with psychological pain. Those who self-mutilate do so in order to punish themselves, express internal turmoil, and reduce severe anxiety