The issue is that of avoidance. If I have to tell an app I've been bad, I'm more likely to avoid telling it at all. (Like not getting on the scale when one knows the weight won't be good news.)

Sure, highly motivated users will track everything, but it doesn't take much to leave the highly motivated state. There's also the angle that knowing you have to report in with bad news might stop you doing the bad thing in the first place, but when the penalty for not reporting in is nil, it loses some of its power. Perhaps approaches that use human accountability or other reward systems (eg. GymPact) could help here.