Sometimes moderators (none of whom have permission to spam clean) come across spam and helpfully report it and move it to the spam forum. We consider it important to remove spam from public view ASAP which is why we have instructed them to do this.
However, when an admin (the only people with permission to spam clean) comes along to properly clean the account up, they can't because the thread is already in the spam forum and trying to clean it causes an error.
It's a pain to have to move the thread out and then spam clean it.
Better if the spam cleaner doesn't try to move the thread if it's already in the spam forum.