The reasoning behind it is multiple:
- the moderators do not like modification rights to any and all reporters because of several reasons, including security and recording.
- the moderators will want to examine the thought process behind a bug report, they will look at the notes to determine what is the current status
- If you were completely wrong you can ask to close the bug report and open a new one with more precise and to the point descriptions
I actually think that is quite reasonable. E.g. I submitted a patch too soon a few days ago, but I got feedback and tried to remedy that: the moderator can follow my thought process and although that is annoying to the moderators or the assigned developer, they can see that the patch(es) were written in good faith and best intention.
So besides the first point, it actually adds value.