Why does my device turn Wi-Fi on by itself?
After some searching, it turns out this feature is provided by the Sprint network (although that doesn't rule out that other carriers/devices may provide similar functionality). The Q&amp;A at community.sprint.com points to the Sprint Connection Optimizer, AKA the "Intelligent Mobile Network and Wi-Fi Setting." This setting "will automatically connect you to Wi-Fi networks on your device's remembered list."

This setting can be enabled/disabled by navigating to Settings > More settings > Mobile Networks > Sprint Connections Optimizer.