God can decree an action that is sinful for a human to perform, because he decrees it for non-sinful reasons.
A sin is only sinful because of the attitude of the heart in doing it. When humans sin, we are by definition rebelling against God. But in ordaining human sin, God doesn't rebel against himself. Rather, he ordains our sins with good ends in mind, which makes the act of ordaining them not sinful, since the attitude of his heart is not rebellious but righteous.
Some biblical expressions that seem to support this understading are Genesis 50:20 and Romans 11:32.
Why did God allow injustice in the first place? listen to one whose insight and understanding of these things is far beyond mine, Jonathan Edwards, answering the question why a good and holy God would decree that there be hardening and evil. Listen carefully. Think hard. This is not the Bible. This is a man who I believe understood the Bible correctly on this point:
"It is a proper and excellent thing for infinite glory to shine forth; and for the same reason, it is proper that the shining forth of God’s glory should be complete; that is, that all parts of his glory should shine forth, that every beauty should be proportionably effulgent [=radiant], that the beholder may have a proper notion of God. It is not proper that one glory should be exceedingly manifested, and another not at all...
Thus it is necessary, that God’s awful majesty, his authority and dreadful greatness, justice, and holiness, should be manifested. But this could not be, unless sin and punishment had been decreed; so that the shining forth of God’s glory would be very imperfect, both because these parts of divine glory would not shine forth as the others do, and also the glory of his goodness, love, and holiness would be faint without them; nay, they could scarcely shine forth at all.
If it were not right that God should decree and permit and punish sin, there could be no manifestation of God’s holiness in hatred of sin, or in showing any preference, in his providence, of godliness before it. There would be no manifestation of God’s grace or true goodness, if there was no sin to be pardoned, no misery to be saved from. How much happiness soever he bestowed, his goodness would not be so much prized and admired, and the sense of it not so great...
So evil is necessary, in order to the highest happiness of the creature, and the completeness of that communication of God, for which he made the world; because the creature’s happiness consists in the knowledge of God, and the sense of his love. And if the knowledge of him be imperfect, the happiness of the creature must be proportionably imperfect. (Jonathan Edwards, "Concerning the Divine Decrees," in The Works of Jonathan Edwards (Edinburgh: Banner of Truth, 1974), p. 528 )"