Nobody likes feeling stuck. And Kay Ryan's titular turtle is "often" stuck. All she asks is the freedom to move (see "Mobility Imagery" in "Symbols, Imagery, and Word Play") so she can find food. But that freedom is blocked at every turn—by geography, by predators, by chance. How can she stand it? How do you stand it when you feel stuck? For humans, freedom is mental as well as physical: even if our bodies are confined, we value the freedom to think, hope, and dream.
Eye of the tiger—er, turtle, gang. "Turtle" suggests that survival of the fittest is the dominant principle of human society as well as the animal world. Only the strong have the power to control their own lives, while the weak have little, if any, freedom.
"Turtle" suggests that people create their own prisons of low expectations. When individuals fail to exercise their imagination and freedom of choice, they surrender control over their own lives, becoming as powerless and vulnerable as a turtle on its back. No fun!