They probably thought that when a new user is created, then it’s most likely that they’ll add a mailbox for that user right away. So they combine that and give you the mailbox without hassle.
Maybe this is true most of the time for most of the customers, and if you don’t need the mailbox then you’re the exception. Of course I don’t have the stats so I’m just guessing here.
They could also make the “Add new user” page in a way where you can select by checkboxes and radio buttons:
X - do you want mailbox?
X - do you want FTP?
X - do you want Samba?
X - do you want shell?
O - /bin/bash shell
O - /bin/tcsh shell
O - /bin/ksh shell
O - /bin/zsh shell
… but they didn’t. Perhaps that would be worth requesting?