It’s not about the FACTS. It’s about the impressions anyone responsible for safety will have between point A and B. Should they have a label or something to alert people trained to assume ALL TANKS ARE PRESSURIZED that they are not? Sure, but if that’s not profitable they won’t do that. Putting it in a box was the correct answer. No assumptions made by safety conscious people = a smooth day on the shipping line.
It’s a good thing they make such assumptions. But them taking time to pull something suspicious out, investigate it and realize that technically the customer is correct - is unprofitable and will delay your packages arrival. Same thing for the people who pack the holds of airplanes.
Its not how you would run an airline, nor me if I could help it. It’s not how it would go in any system where personalized attention was a rational expectation. But that’s neither TSA nor United Airlines, they’re not going to treat our stuff as special, because it isn’t. Our stuff is equivalent, like us!