In other words, it's WT's position that DL is not at fault because a guy carrying a duffle bag filled with semi-automatic rifles didn't raise any suspicions from other DL employees.
The letter of the law says DL is accountable and responsible for securing their leasehold.
If someone's walking around the ramp without a visible SIDA badge and it isn't questioned, DL is the one who gets fined. I'm told our corporate security auditors will walk the company's premises systemwide without visible ID just to see if people challenge them (and not just at the airports, but also in the HDQ complex).
If DL's employees haven't been trained to look for suspicious behavior like this, then ultimately, they failed.
BTW, a union has a duty to represent the employee when disciplinary action or charges are filed against them by the company. It gets a little muddy when criminal charges are filed due to actions unrelated to their job duties.