Why does my MobileMe email account have a Junk mailbox?

Question

My MobileMe email account has a mailbox named Junk (first letter capitalized), and I don't know why. I created a mailbox called junk (all lowercase) into which I manually move junk messages. I see both mailboxes everywhere I view the account (Mac, iOS, me.com web interface).

I use one Mac with the MobileMe email account, and junk mail filtering is disabled. I also use the MobileMe email account on an iPhone (iOS 4.3.5), an iPad (iOS 4.3.5), and occasionally the me.com web interface. Within iOS settings, I don't see any setting specific to junk mail filtering. I have no mail rules that move messages to a Junk folder on my Mac or on the me.com web interface.

Something is automatically moving messages to the Junk folder, and is getting some false positives with its junk detection, so I want to stop that.

How do I find out what's responsible for this mysterious Junk mailbox, and prevent it from automatically moving messages identified as junk? I am trying to get rid of the Junk mailbox and only manage junk messages manually. I have deleted the Junk mailbox in the past, and it came back, although I don't remember the specific circumstances related to its reappearance.

I've already viewed this knowledgebase article (which offers the opposite of what I want): http://docs.info.apple.com/article.html?path=Mail/4.0/en/15591.html