"All things whatsoever ye would that men should do unto you, do ye even so unto them, for this is the law and the prophets."
The Golden Rule is the highest law; it is the definition of the behavior of Love. The rest of the Sermon on the Mount are pedagogical examples of how to apply this law because by our fallen and carnal nature we do not possess Love. The Sermon is not an exhaustive list of all the ways to do good and to love all, and by the nature of this law it cannot be made exhaustive.
How do you refuse the higher law?
You do not believe it when it is set before you and you do not do it of your own free will and choice because you do not believe in Christ nor love him, but you do believe in and love the power of money, the power of debts, the power of ostracization and threats thereto, the power of spin or judicious deception, the power of the myriad ways to be unpleasant to someone to get them to bend to your will.
Or maybe you simply think it's too extreme to be true. Surely God doesn't intend for us to do things that simply don't work, one may think. But that's just another way of saying what was said in the preceding paragraph.
So God may offer something else so that those who will be saved may be. A teacher, a schoolmaster to bring those few who actually do believe unto Christ.
The law of the Church in the D&C, given to a people who had the Sermon before them, looks an awful lot like the 10 commandments, doesn't it?
There's a reason for that.
On the other hand, King Benjamin's sermon, given to those who were under the Law of Moses, bears a strong resemblance to the Sermon on the Mount.
There's a reason for that, too.
Of the two groups, the Church and the people of King Benjamin, which one was on the way up? And which one was on the way down?
Which one do you wish to emulate?