Every culture has it's taboos.
It's best to be aware of them before you visit.
What the bible guys do is worse than nude sunbathing.
A U.S.A. equivalent might be masturbating in public.
Leaving Bibles in North Korea makes these guys feel good.
Then they'll wait in North Korean prison for some secret or not-so-secret ransoms to be paid. North Korea does that with everyone, even holding the nation's own starving populace hostage.
The sad thing is these Christian evangelists will puff themselves up further in the name of God, accepting their persecution as some kind of religious test, their Holy time in the desert.
The only ethical way to spread a religion, any religion, is by living it, without violence, without overt hostility or lording it over others.
The "I'm here to save you!," and leaving Bibles in hotel room Christians, that's standing on corner and praying in public so that others may see you, and it's not even By-the-Book Christian.
But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly. Matthew 6:6
I don't think it's proper to fault people for traveling anywhere, even dangerous places, but it's always best to know the rules of a place before you begin. What this guy did is the equivalent of swimming in the pools above the waterfalls of Yosemite, or playing with the Grizzly bear cubs. Bring a Bible into North Korea and the odds are good something bad is going to happen to you.