Their back door is likely a security door, so you can still open it when it's "locked" but when you do an alarm goes off... yknow in case you're escaping a fire or something, and blocking it with anything is an OSHA violation.

Yes because it still blocks an exit

Absolutely. Locked exits are also a safety violation. Check out the history of the Triangle Shirtwaist Factory. The owners of that building kept the doors locked while the women worked there (claiming, iirc, that they wanted to protect against "theft") and when there was a (very famous, devastating) fire, many, MANY women and young girls died (whether they were burned, suffocated from the smoke, or jumped from the windows to a faster death). Remember-- safety regulations are written in blood.