That's because the Desire S is unrooted by default; no user can delete the OS and stock features, unless they root the phone, i.e. enable complete access to the system files.
It's one way of ensuring the network provider/HTC less hassle from people who delete things by mistake and then find the phone doesn't work as before. They're also a way of branding the phone.
It's now possible to root the Desire S, but there are risks involved for the uninitiated.