This is too funny! I just posted a question tonight that had me pondering the verse "...Mercy triumphs over judgment" (James 2:13) today. Good question.
Like you, I tend to be fiercely principled and on the "justice" side. It's not that I'm an uncaring person - honest! - but I think it's because I have found from experience that...
1. Relativity is a slippery slope and makes it very difficult to draw the line on things, esp moral issues. I find it easier to live in the black and white and know where I stand. I'm not always in the right, but at least I know it!
2. Too much "mercy" (if there is such a thing) can sometimes lead to people taking advantage of you, which, again, makes it difficult to draw the line and set boundaries when they're needed.
3. Lack of "justice" / accountability sets a precedent. If people know there is a hard and fast policy with regards to something, they are less likely to take advantage and encourage others to do so. Depending on the situation, this can lead to chaos.
4. My personality style (first born, type A, ENTJ...J for "judging" rather than "feeling"...ouch!) functions especially well in situations with clear boundaries, communication, and accountability. It should be noted that I hold myself just as much to these things as I do anyone else. I know there are other personality types that are more intuitive and feeling that find the same comfort in the grey areas.
All of this also depends on the context. I know many out there are questioning of authority, etc, but the fact is that there is a natural hierarchy in most things that needs to be honored in order for chaos not to rule. I'm learning to temper my need for "justice" with mercy more and more, but I have to be very intentional about it.