The scripture never explains why he was permitted to do something against the law, but can anyone deny he was given that permission?
Naaman the Syrian was healed of his leprosy when he reluctantly obeyed Elisha's instruction to wash himself seven times in the Jordan whereupon he believed God. (2 Kings 5)
And he returned to the man of God, he and all his company, and came, and stood before him: and he said, Behold, now I know that [there is] no God in all the earth, but in Israel: now therefore, I pray thee, take a blessing of thy servant. - 2 Kings 5:15
Jesus our Lord gave the incident with Naaman as an example of the Spiritual Truth that prophets are rejected by their own people.
And he said, Verily I say unto you, No prophet is accepted in his own country. But I tell you of a truth, many widows were in Israel in the days of Elias, when the heaven was shut up three years and six months, when great famine was throughout all the land; But unto none of them was Elias sent, save unto Sarepta, [a city] of Sidon, unto a woman [that was] a widow.And many lepers were in Israel in the time of Eliseus the prophet; and none of them was cleansed, saving Naaman the Syrian. - Luke 4:24-27
This also points to the Gospel of Christ being rejected by the Jews while being received with gladness by the Gentiles. (Romans 11, boast not against the branches) And that is a fulfillment of this prophesy from the Song of Moses:
They have moved me to jealousy with [that which is] not God; they have provoked me to anger with their vanities: and I will move them to jealousy with [those which are] not a people; I will provoke them to anger with a foolish nation. – Deuteronomy 32:21
The Song of Moses will be sung in heaven along with the Song of the Lamb:
And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous [are] thy works, Lord God Almighty; just and true [are] thy ways, thou King of saints. – Revelation 15:3
And again, Scripture speaks of the fulfillment of the prophesy:
I say then, Have they stumbled that they should fall? God forbid: but [rather] through their fall salvation [is come] unto the Gentiles, for to provoke them to jealousy. – Romans 11:11 And other sheep I have, which are not of this fold: them also I must bring, and they shall hear my voice; and there shall be one fold, [and] one shepherd. – John 10:16
Where are you reading that Naaman was permitted to do something against the law? My leading in these passages is that Naaman's spiritual life story was in fulfillment of the will of God and directly related to His promise, standing as an example for our illumination.
To God be the glory!