You can't delete the root site, but it is possible to delete secondary child sites. Its disabled by default to prevent accidental deleting a site. Once you delete it there is no way to get it back, so it seems wise in most cases to keep it disabled and just enable it when you need to delete sites.
So if you want to enable deleting sites, look for this setting in Web.config:
<add key="AllowDeletingChildSites" value="false" />
and change it to true.
Hope it helps,