iCloud mail setup in 10.8 become another IMAP account

I'm using 2013 MBA w/ 10.8.4, and just setup my iCloud account.

After login my Apple ID, everything is fine, Mail, Notes, Contacts, etc setup correctly. All itmes under iCloud setting is checked and work fine.

But once I open Mail, the Mail under iCloud Preference will unchecked, and a "iCloud IMAP" will added into Mail Preference.

The Mail work fine, but I think the Mail under iCloud should not be uncheck.

Is there any thing wrong?

How can I fix this?