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"