Craft Commerce Required Address fields
You can make address fields required in two ways - at the front end using your own js form validation or html techniques (and note that the Commerce example templates are not at all intended  for use with live stores, they're just examples to get you going with your own development).

The second approach is in a plugin -

Craft Commerce: Validation on Address Fields

...the ideal approach is probably to use both methods.

Currently (Dec 16) there isn't another way.  I presume this is to give maximum flexibility with checkout and perhaps (and hopefully) pending the time when addresses become proper element types with custom field support.  That would be nice.