Even though some stories do have a happy ending, it doesn't seem to me that the tales from this book have any serious commitment to that. I have also noticed that stories like The Frog Prince, Rapunzel and The Sleeping Beauty are told in completely different ways in recent times in order to entertain rather than cause deep thinking.
It is important to say that not being seriously commited to entertainment and happy endings is not a bad thing. Grimm's tales are told in a way to make the readers think carefully about each story to understand their meanings and their connections to the reader's life. In those tales most characters have very human behaviors, even some animals do. The kings, queens, princesses and princes are not perfect people: they make mistakes, they get angry, they are sometimes unlucky despite all their wealth. The king from Faithful John tale is an example: he kidnapped his lover, which is not any laudable action, and was very unlucky after that. The ending was happy in this case but the young king's life was not all beauty and perfection as it is usually presented in many tales about kings, queens, pincesses and princes told nowadays. In my opinion, this specific fact makes Grimm's tales a lot special as it is possible to put ourselves in the characters shoes.
In real life many times mean people win and nice people are just fooled, as told in Cat and Mouse Partnership. Many times money is what matters above many other virtues one may have; many times beauty means more than character in love, as told in The Frog Prince, for instance. I believe that is what Grimm's tales are about: real life. This is what they are commited to. Those tales can make us undestand some truths about real life through simple and short stories.
