Why did thoreau come to the conclusion that the state was half-witted when it imprisoned him?

It was because it was not courageous and did not know its enemies from its well-wisher friends and physical consequence wasn't something that discourages.

In 1846, Thoreau happened to meet the local tax collector, Sam Staples, who solicited him to pay six years. Thoreau denied on account of his disagreement to the Mexican - American War and slavery and he spent a night in prison in light of this refusal.