Just because fantasy is a genre that demands we suspend our disbelief and embrace the impossible, it doesn't mean that the worlds it takes us to aren't still bound by logic and common sense.
Swords and sorcery and all well and good, but at its heart, fantasy tells us stories that we can relate to, and that means characters with flaws and failings just like the rest of us, even in a world fundamentally different to our own.
It's usually quite easy for a questionable decision to be explained with a little clever writing - like Tolkien having the Ring corrupt everyone, compromising their ability to act with any sense - although there are times when all common sense clearly goes out of the window.
Poor decisions in movies are usually made in order to further the narrative in some way, and to that end, these entries certainly made for entertaining viewing, even if they did make the audience groan at the sheer stupidity on display.
Whether these decisions were made based on greed, curiosity or misplaced trust, the simple fact is that each and every one of them was very, very dumb.