Genesis:
Anyone who picks up a tank and connects it to their regulator without KNOWING what is inside it deserves what they get.
Contents labels are affixed right near the valve on the neck of the tank. You'd have to intentionally not pay attention to miss it.
Clearly you have never been in a setting with a large number of students, all at various stages of the OW course, with the associated buzz of activity, trying to keep things moving as any good DM should do:
"Your in next, have you got your log book filled out, do you know where your buddy is your with the next group, have you changed tanks yet, what tank pressure are you starting with." Then the students add: "Where's the washroom again, do we have time for a snack, I here a funny sound from this reg is it normal, my fin strap just broke their waiting for me in water right now do you have a spare, I think my o-ring is missing, can my wife swim with us while we take the course"........and a thousand other questions, problems, and little events, all happening at the same time!
So if in all this confusion, the student who is in a rush to get ready for dive # 2, picks up a tank and fails to look for that little insignificant contents sticker, because they are just overwhelmed by all the excitement and new experience. Connects that tank to their reg and uses it. It's too bad for them, cause they are just stupid. Is that what you are saying? In the real world, people have faults and make mistakes, and some people take longer to learn than others. In the real world we try to remove as many possibilities for errors, in as obvious a means as possible. This means we put an easy to recognize, easy to see label on any tank that contains a compressed gas other than air. If this simple concept is lost on some people, and no amount of logical rational explanation will convince them that this is a good idea. Then I only hope they are never in a position of instruction or guidance. If they are, I pity the students.