Tldr for safety
To actually answer your question instead of piling on, it's a hospital, not a prison. In case of emergencies, the door absolutely cannot ever be potentially locked, even while the machine is on.
With how easily something can go wrong in an MRI, they need quick access without the addition of special keya/badges to get inside or relying on people inside to hit some lock release.
In cases like this it makes perfect sense to have a lock because an idiot was outside and ignored all the warnings. A lock would have prevented everything that followed him entering.
Buuuuuuut unfortunately we can't cater the entire world to the biggest idiots, if only for the safety of the less idiotic who might have a heart attack in the MRI and need to be quickly pulled out, or a piece of metal that snuck into their food and is now ripping out their insides.
In most situations where an emergency happens inside, quick reactions save lives, and locks slow reactions down to the slowest mechanism, which might be "I don't have the right RFID badge, go find another person who has one or the guy inside dies"
There was on one that I've been in, not sure about this one.
From my understanding, when an MRI is emergency stopped it doesn't stop immediately, and it causes a lot of damage, so staff are less likely to use it in an emergency. Stupid, yes. But when you're worried about getting fired for hitting a button, you're less likely to think of a situation as an emergency. You would think "chain strangling a man" constitutes an emergency though...
As for the staff not stopping the guy making a beeline for the door with more than just words, I'm not sure. I would prefer staff tackle me to the floor rather than let me blithely walk to my doom. Of course I'm only in my 30s...
The hospital is absolutely partly to blame, especially if they didn't properly convey the danger beforehand. All 3 hospitals I've recieved an MRI from have been pretty insistent about making sure I have no metal on or around me before I go in the doors though.
I'd say it's about 60/40 on the hospital.