So that they will not be accused of any form of sorcery in relation to the sickness of Mr. Welsch.
Explanation:
In the book "Cultural Anthropology" Robert L. Welsch and Luis A. Vivanco discusses the various and different approaches to sickness and diseases, depending on the environment and culture of the people concerned. For example, when anthropologist Robert Welsch contracted malaria while doing some field work among the Ningerum people of Papua New Guinea, the villagers want to take him to the health center that was miles away. Even though he tried the American way of dealing with the sickness, by taking fluids and medicines, seclusion and lots of rest, the villagers, as is their custom, did not let him be alone. Instead, they have a completely different perception of their sicknesses. They believed that illnesses are caused by sorcery, and so they tried to comfort Welsch and stay with him for long hours. They were scared of being accused of doing some sorcery on him so rather than being wrongly accused, they would rather send him to the health center where they won't come to any sort of harm.
I would say that William Blake directs the poem's praise to the gentleness of nature. The book where all these poems are written is called Songs of Innocence, so it is obvious what he is talking about.
