Chernobyl isn’t safe safe, it’s just safe enough for wildlife to survive there, possibly with lowered life span and quality of life.
Also, there’s a decent danger of radioactive dust coming off the book if it’s handled. It may not be that radioactive, but if it clings to you, or you breathe it in, it will do considerably more damage than if it was all one solid rock that made geiger counters click.
Personally, I prefer duplicate keys to be eaten by the parser but I can see how it’d be beneficial to prevent them.