The idea, "it's fiction; it doesn't have to make sense" shows a lack of critical thinking skills. It says that you can't predict likely outcomes from given circumstances and premises... when that's how all of science works. It also shows a lack of imagination, a lack of ability to posit circumstances different from the ones that exist right now--because the idea that all not-factually-true situations are equally likely and believable is only possible for someone who never considers possibilities.
If a book snaps your disbelief suspenders, it could be that you're reading it from a different mindset than intended for the enjoyment of the book. (If you're gonna enjoy romance books, you have to accept the concept of instant-true-love-happy-ever-after, no matter how much real life tells you that it doesn't happen like that.) But it could also mean that the author has failed at worldbuilding--that she hasn't explained why Thing X works when Things Y & Z don't.