"The Lord said unto Enoch: Behold these thy brethren; they are the workmanship of mine own hands, and I gave unto them their knowledge, in the day I created them; and in the Garden of Eden, gave I unto man his agency;
And unto thy brethren have I said, and also given commandment, that they should love one another, and that they should choose me, their Father; but behold, they are without affection, and they hate their own blood" (Mos. 7: 32-33)
If Satan's plan had been accepted, he would have given people choices over what clothes to wear, which restaurants to eat at, whether to go skiing or mountain biking, and every other eternally meaningless decision elevated by capitalism. He would do this to hide the emptiness we would have in the place of our hearts, which would never know love or hate, never know anguish and happiness.
The agency God protected was our fundamental ability to choose love or hate, good or evil.
Taxes can neither increase nor decrease our God-given agency, because moral agency is not proportional to our economic means but is defined by our contextual decisions about love and hate, good and evil.
Even if a tax to support health care seems to be forcing us to do good, the triumph of God's plan means we will still be left with the vital moral choice alluded to in Moroni 7: 8.