Would that be surprising to you? I assume you might be an American - I was really surprised when I visited the US and noticed that most people use unlocked boxes located outside of their houses to get their mail. Where I live people either lock their mailboxes or they get mail through a slot in their locked door - nobody leaves their mail accessible.
Unlocked mailboxes outside the house really only exist in rural areas or regular suburbs (to be fair, these sort of homes probably make up a plurality of homes in America).
The bigger problem is what happens with packages. Unless you live in an apartment building with a staffed leasing/mail office, chances are your packages get left by the door (usually in plain site), or you need to be home to sign for them.
http://boston.cbslocal.com/2011/12/20/two-arrested-for-theft...