I think it comes from a commitment to nomic reductionism. The Second Law is, well, a law. But if you really believe that laws are rules, there is no room for autonomous laws at non-fundamental levels of description. The law-likeness, or "ruliness", of any such law must really stem from the fundamental laws. Otherwise you have overdetermination of physical behavior. Here's a rhetorical question taken from a paper on the problem: "What grounds the lawfulness of entropy increase, if not the underlying dynamical laws, the laws governing the world's fundamental physical ontology?" The question immediately reveals two assumptions associated with thinking of laws as rules: the lawfulness of a non-fundamental law must be "grounded" in something, and this grounding can only conceivably come from the fundamental laws.

Yes. One might worry that the second law, which is clearly not fundamental, doesn't seem to be grounded in a fundamental law. The usual solution to this is to realize that we are forgetting an important fundamental law, namely the boundary conditions on the universe. Then we realize that the non-fundamental law of entropy increase is grounded in the fundamental law that gives the initial conditions of the universe. I don't think this is "[coming] up with an elaborate hypothesis whose express purpose is accounting for why [the second law] is lawful," as you seem to imply. Even if we didn't need to explain the second law we would expect the fundamental laws to specify the initial conditions of the universe. The second law is just one of the observations that provide evidence about what those initial conditions must have been.