It doesn't - what it does, is it jumps to the first account that was set up. As most of us set up our iOS devices, we have to log in using an Apple account; at that point, it sets up iCloud mail for you (whether you want it, or not).
So, even if you subsequently add extra mail accounts (such as during a restore, or manually) they will always be the second, or third, or fourth (or however many) you set up.
Disabling iCloud mail doesn't remove this "switch to" behaviour, it just means iOS will switch to the first account in the list now (which won't be iCloud, if you disable its mail handling component).