Okay I see now that this is default vB behavior in vB 4, when clicking the "+ Post Reply" button, rather than using Quick Reply. This is really annoying, because in my opinion, if someone wanted to Quote the post, they would have clicked "Reply with Quote".
As far as I can tell (checked vBulletin Settings and Forum settings), it's not possible to disable this "functionality". I suggest you post a report with vBulletin if you want to find out or get them to add a way to disable this.
I tried doing a search over there to see if anyone else was bothered by this, but vBulletin.com doesn't let you search for the terms 'quote' or 'reply'.