Which is no different from how any other religion shapes the Bible to fit its own viewpoint and theology. If any given religion doesn't like the way a certain scripture doesn't seem to fit into its theology, then it comes up with an interpretation that bends, folds, spindles and mutilates the meaning of that scripture until it does fit.
You're just unhappy that Joseph Smith claims authority and permission from God for doing so. No number of appeals to various Greek manuscripts is going to be able to discount that claim of authority. The office of prophet isn't a popularity contest where the outcome is voted upon by counting the various translations of Greek manuscripts.
Here's what Nibley has to say about Joseph's "translations":
We must not think that the Lord in giving his servants special devices to assist them was letting them off easy. He did not hand them the answer-book but only a slide rule. It takes far more formidable qualifications and far more intense concentration and cerebration to use a seerstone than it does to use a dictionary; the existence in our midst of computers does not mean, as some fondly suppose, that mathematicians and translators and genealogists no longer have to think-- they have to think harder than ever. A Urim and Thummim, like a dictionary, is only an aid to the translator who knows how to work it, and may be gradually dispensed with as he becomes more proficient in his spiritual exercise. Admittedly, translating with a Urim and Thummim is not the normal way; it does not require philological training, but training of a far more exacting sort, since like the seer-stone it shows "things which are not visible to the natural eye" (Moses 6:35-36); it operates, as Buckminster Fuller would say, by the mind and not by the brain. That requires even greater effort and discipline: "... when a man works by faith," said the Prophet, "he works by mental exertion, instead of exerting his physical powers." It is the exertion of the mind, and it is the most strenuous and exacting work of all. Certainly the documents with which Joseph Smith was dealing could be translated in no other way than by the Spirit. How can any mortal ever know what the original first writer of Genesis had in mind save by the power of revelation? And without that knowledge no translation is possible. It was Brother Joseph's calling to interpret the minds of other dispensations to our own, and during the short time in which he worked at it he covered an astonishing lot of ground, handling huge masses of material which could only be rightly understood and explained by the power of revelation. In fulfilling his formidable mission he was never bound to any particular method or text or vocabulary or rules of grammar, since they are merely aids to any translator's ignorance. Every good translator will tell you that after all the aids and implements at his disposal, including his own long training, have been brought under contribution, it is in the last analysis his own feeling for things that makes a convincing translation-- without intuition he could never make it. If truly scientific translation were possible, machine translation would have been perfected long ago; but where wide gaps of time and culture exist such a thing as a perfect translation is out of the question: in the end it is the translator's own imponderable intuition that is his claim to distinction. The most learned linguists do not make the best translators, and the uncanny skill of a Scaliger, Hicks, George Smith, or Ll. Griffith could divine the meaning of texts before which science and scholarship were helpless.