One of the services Apple generally provides is that a phone is locked to a given Apple ID, such that if you wipe it, it still knows it belongs to a given owner, and you need to unlock that for someone else to activate it. It wouldn't be unreasonable to suggest Apple would want Find My iPhone to work even after it's reset.

That being said, my theory is in another comment.

Can you even imagine? Lost your phone? Did you make sure that you were logged in before you lost it? Did the thief reset the phone and log you out? Oh, guess you can't find your phone now. Sorry.

Once you’re signed in you stay signed in and you can’t sign out without authenticating. Resetting the phone doesn’t bypass this.

Yes, when your phone is locked, but only if you've logged the phone into your iCloud account.