If it's ok to be so weak that it's trivial to enter like this, why have the lock at all? The cynical (and probably accurate) answer is that it's security theater designed to give customers warm fuzzy feelings cheaply and with low risk of lockout calls and maintainance issues. It does basically nothing to keep someone from taking your stuff.
It's weak, but I don't know if trivial is the right word. Like a bedroom lock, it does what it needs to do. First, the lock is probably there to make it easier for one remote security guy to monitor many locations after hours. They can be alerted when a door opens, has been ajar for a while and along with a camera, see what's going on. Other than that, you also want a lock to help keep the door closed to help keep climate control working efficiently, keep animals out, etc. Like I had been alluding to, when you rent a unit, you're told that you are responsible for locking your own unit with a specific, harder, lock, like a front door lock. If an attacker can get by this lock, entrance to the premises would have been possible as well.
On that note, I don't think the absence of a super strong lock on the front door of a storage place invites criminals. I live near open air storage facilities where anyone can just walk up to the units, like they're walking by cars on the street or in a parking lot. Thefts from storage units are just not big enough of an issue to require additional measures.
