That was my initial reaction, too. But for an app like Trapster it might not be a bad idea. (I haven't used it, but I guess it's an app for tracking speed traps)

Most apps do nothing in the background. Others, like music or GPS apps, it should be obvious to the user that these apps are doing stuff in the background. For an app like Trapster, however, it would be easy for the user to forget or not understand that it's working in the background. I, as a user, might appreciate this kind of warning from an app that I normally would not want running in the background.

By default, I would have it not do anything in the background (which is how it obviously works on OS 3). I would have a setting that enables it to run in the background, as well as a setting that enables the warning. It sounds like that's exactly what they're doing, although I don't know what their default behavior is.