Lets approach this from an in character, roleplay perspective. Lets say that I am an assistant, who has just shocked himself while trying to hack the hot drinks vendor. My hands now charred stumps, I rush to medbay, walking in as some other patient walks out. Rather than asking for a doctor to fix me, I jab my burnt digits at the medicine locker, pulling out a patch. I apply it to my burnt hands, which regain some of their original color and shape. Seeing that no one has stopped me from accessing the medical locker, I decide to take a few other patches for myself, as well as some painkillers so I can get high later. Because no one has stopped me yet, I decide to steal the CMO's cat, because apparently it's just free game in medbay today. However, the medical vendors scattered around medbay stop my looting spree, as they have ID locks on them. I scratch my head and think, *Why did they bother to put locks on these things if the fridge is free game?*. I wander out, my bag full of pills and patches that will likely never be used by someone who actually needs them.
Alright, lets break down this little episode, and hopefully I'll get you to see why the fridge being unlocked is so ridiculous.
Anyone can treat themselves. Doctors? Pffft, who needs them. Anyone can undermine the doctors by throwing themselves into a cryotube or stealing some patches from the vendor.
Anyone can help themselves. If the fridge is unlocked, of course people are going to steal from it. The fridge and all of it's goodies are in an easy to reach spot, free for anyone to take. It's a blank check for powergaming. Why not steal a few synthflesh patches? Players who resist powergaming will stop themselves from stealing all the synthflesh patches for themselves, but with our high population it's not logical to assume that every player will abide by an unspoken rule.
The vendors have locks, while the fridge doesn't. Why? Who knows. I can't imagine a doctor allowing a patient to take a bottle of medicine back home with them in real life.
Stopping medicine thieves isn't easy. Unless a doctor with a syringe gun or the CMO with their baton is nearby, detaining a thieving assistant isn't easy. Stealing from the fridge is as easy as waltzing into medbay, rapidly clicking on what you want, then hitting the door button and escaping into the hallway.