Because the OS shouldn't be making assumptions about applications' interaction models. People tend to write apps that OS creators never dreamed about. There are legitimate reasons for resource usage (camera) to continue without user interaction.From the top of my head, I can imagine writing a baby-monitor app that uses camera to stream video to my phone in the other room using wifi.

The resource-limiting heuristics have to be impossibly good, otherwise it gets in the way of useful apps.