It's not like writing people off and buring bodies are similar. I can't believe the idea of writing someone off is that foreign to some that it would appear so.
This is where the democratic nature of language fails us. The vast majority of human beings will stay in relationships that are not beneficial or even destructive out of some sort of misguided notion of "obligation", however they dress this notion up. As a result, all terms that refer to removing someone from your life, once you've assessed their involvement to be of no benefit or detrimental, have negative connotations. Simply put, there's no way of phrasing the notion of "writing someone off" in a positive light even though doing so can be the best and most positive thing you could do.
I think people get upset when they're written off because maybe they feel like the relationship is worth saving, they don't view you (general you) as just taking up space, and they're surprised when the other person doesn't feel similarly. Sometimes people stick out of obligation, but I find that's not as true as you think. If it's like Mycroft said, it's not destructive or beneficial, really not doing anything, what difference does it make for the relationship or person to still be there?
I look at it like this: I have a lamp sitting in the corner of my room that's just been there for more than a year. I keep looking at it thinking I should throw it away but it's not obstructing my path so it just stays there. Maybe one day I'll need it, maybe I'll throw it away this weekend. I don't know how my relationship to the lamp will change over time, and it's not harming me so it's no problem for it to just be there.
This is why I think people tend to react so strongly to INTJs. There is a mentality that people either need to serve a purpose and use or they should be removed. The INTJ thinks how do they relate to me, instead of asking how do I related to them. There's no joy of having people around just for them to be there. This can be (mis?)construed as misanthropy. Be useful or leave. Maybe they do serve a purpose, but you don't know what it is yet. IMO, to sever the relationship is foolish simply because you don't know what may develop (for your happiness or the other person's).
I think INTJs think they're being thorough in the possibilities but they continually miss the people aspect. This is also related to PR, people may see an INTJ treat others like this and when approached by an unsuspecting INTJ they react without knowing what the nature of the transaction will be. This is part of the intimidating aspect, a person thinks, they'll just cut me loose when they're done with me, why should I help them? People sense this attitude and they react very strongly to it, often not in a good way, and even more often antagonistically to what you're trying to do. Combine this with the INTJ trait of being standoffish and cool and it's just a mess. This then becomes a vicious cycle between INTJs and the rest of the population. Maybe this is why I take particular pleasure in annoying INTJs

. Some kind of convoluted ass-backwards notion of helping them see that people don't need to just be a tool for the implementation of goals.