Musar for Parah 4:8

Rabbi Joshua of Sakinin in the name of Rabbi Levi in Bamidbar Rabbah 19,5, says that there are four laws in the Torah described as חק which Satan focuses on when he wants to undermine our faith. One is the law that although one must not marry a sister-in-law, the legislation of the levirate marriage contradicts that principle. Another is the legislation of כלאים, the mixing, cross-breeding or grafting of certain fabrics, animals or seeds. A third is the scapegoat that is killed on the Day of Atonement; the fourth is the legislation involving the use of the ashes of the red heifer in our portion. In connection with the prohibition of marrying a sister-in-law it is written ושמרתם את חקתי, Lev. 18,5. Nonetheless, this prohibition is set aside when one's brother had died without children. The exception to the law of לא תלבש שעטנז, in Deut. 22,11, not to wear a mixture of wool and linen, is the permission to override that principle when wearing such a mixture as part of a prayer-shawl, טלית. The exception to the principle of the ashes of the red heifer purifying a person, is that the clothing of the person or persons who actually are involved in the procedure of preparing these ashes becomes defiled even though in a state of ritual purity (19,8). The ash of the red heifer itself purifies impure clothing. There is, of course, a lot that can be queried about the whole procedure of the scapegoat. Why did Rabbi Joshua not mention other instances where the expression חק is used, without contradictory aspects to such acts of legislation?