Why would Joseph Smith not include the “without a cause” part of Matthew 5:22?

In the Book of Mormon, Jesus says in Nephi 12:22:

But I say unto you, that whosoever is angry with his brother shall be in danger of his judgment

This is slightly different to Matthew 5:22 which says

But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment (emphasis added)

Why would Joseph Smith not include the “without a cause” part?

Michael R. Ash explains:

When we examine the earliest Greek copies of the New Testament –documents that were discovered after Joseph Smith had died –we find that the phrase “without a cause” is also generally absent. As Professor John Welch notes, the verse in 3 Nephi discourages all anger whereas the verse in Matthew permits justifiable anger. Some non-LDS scholars believe that “without a cause” was added to Matthew 5:22 during the translation process, while the Book of Mormon more accurately reflects the likely original intention of the passage. The fact that Joseph Smith got it right, when no scholars in his world would have been aware of the later Greek insertion shouldn’t be amazing –but it is
– Michael R. Ash – Faith and Reason 20: “Without a Cause”

How would Joseph Smith have found out about the earliest Greek copies of the New Testament?

See:

Michael R. Ash – Faith and Reason 20: “Without a Cause”

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