censoring the truth--fiction, or just plain lying?

so i was stalking browsing through some people's blogs and came across this post on vicki's blog. it was linked to this page on wikipedia regarding hypergraphia. at the end of the article, i came across the line:

the diary was edited by daniel aaron and published in 1985 by harvard university press.

which prompted me to think, why would you ever edit a diary? that seems to me a heinous crime against the author of the diary. sure, they wouldn't want certain secrets being divulged to the public, but then if the diary is published posthumously there is no problem. so why would you ever edit a diary? if you edit a diary, then you are changing what the author is saying--and that is just outright lying. a diary is a good (truthful) window into a person's mind, and changing anything in the diary is almost the same (to me) as murdering that person's thoughts and replacing it with a surrogate which is totally not honest at all. so, why people edit diaries, i might never ever know.