Because just as if you worked as an opening manager for your local dollar store and was given a set of keys to be able to open up shop every morning, yes you were approved and authorized at that time while you were agreeing to work for them. Additionally, having those keys doesn't authorize you to go in any time you wish and take any and/or all items you wish. And had you resigned from that job and still had the keys because there wasn't a chance to return them yet, using those keys to get in is now quite obviously unauthorized, as the day you resigned you relinquish your employment and rights to access it. Even without the keys, if you discovered the door unlocked in the middle of the night, you'd still be charged with breaking & entering, because surprise surprise...the door being unlocked is not necessarily granting permission or authorization for you to go in.