Warning signs, eyeballs, reading, and common sense are sufficient most of the time.

I can easily recall the warning signs for flammable, explosive, poison, and most people I suspect would also be able to recognise the radiation one despite that being the least likely material they'll be handling, but none immediately comes to mind for "dangerously powerful magnet".

"Most of the time" is not good enough.

Door should only open if no metals detected.

Why though? Why are we going to force society to spend hundreds of millions of dollars in equipment, wasted time and personal costs, to avoid a one in a million possibility that someone not caring about clear warning signs gets injured?

If your argument is money, you are overlooking that a metal detector is really cheap, a practically invisible cost when next to that of the MRI machine.

Thus, it makes sense for regulations to mandate such a life-saving metal detector.

It's money, wasted time, and more personnel to guard, handle and fix the metal detector.

I think we have way too many regulations as it is, and it's bringing society as a hole down.

Clearly not. Feels like you'd want a metal detector tied to a door lock.