There can be two explanations for the keyed entry and exit. First, when you have a window next to your door it keeps someone from breaking the glass and then reaching in and opening the deadbolt. The front door of the building where I work is set up this way.
You nailed it. "Double cylinder" deadbolts were recommended (by insurance companies no less) for security in doors with windows/sidelights at one time. This still holds true for commercial buildings, but as far as residential they are largely illegal nowadays in most jurisdictions, banned by fire safety codes. As it turns out, there were large numbers of fire related fatalities and related injuries where double cylinder deadbolts were in use, because panicked residents and those unfamiliar with the house forgot they needed the key to exit (exactly as depicted in the movie).
At some point in the early 90's, the industry reversed course and told people NOT to use double cylinder deadbolts in homes. Residents were urged to replace them with standard single cylinder bolts as soon as possible, and those who couldn't be replaced immediately should either permanently glue a key into the inside cylinder or otherwise lanyard a key to the door in immediate reach of the lock.
Nowadays in most jurisdictions you can't even sell a house that has double cylinder deadbolts. Home inspectors will flag them, and in areas where the fire marshal has to sign off on a sale, it will not pass until they are replaced. A couple of the companies have worked around this by creating 'captive key' systems, where the outside cylinder works normally, but if the door is locked from the inside, the key is locked into the cylinder. IMO not worth the extra trouble or expense, since if a burglar is willing to break a window to unlock the door, they'll probably just climb in the window once they figure out they can't unlock the door, LOL!
This also applies to the security bars that were on all the windows.. I'm not sure what the codes were in the 80's, but today you'd never be allowed to put that kind of permanent bars on bedroom windows. They need to have a latch mechanism that allows them to open from the inside without requiring any type of tool (and as far as fire codes go, a key is a tool).
Their friggin house was a total deathtrap, LOL!
The only other time I saw a setup like that was when I was in middle school. I had a friend who had a "special" uncle who lived with them. The door locked automatically and you needed a key to get out because it kept his uncle from wandering outside and getting lost or in trouble
While I absolutely understand the reasoning, locking someone in their room is a dangerous game. Had there been a fire and he couldn't get out, they could've been charged with reckless endangerment, possibly involuntary manslaughter.
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