The issue here lies within your assumption that people with radar detectors have it to speed. That is simply not correct. Sure, some do. But not all. Just like everything in life. You can't make an over-generalization like that. It's foolish. Some people want added situational awareness. Some like to know if a cop is around as a REMINDER for them. Perhaps they would have lazily changed lanes without a signal, without the reminder. There is absolutely no reason for everyone NOT to have one. Waze is great too. You can report weather, objects in road, and of course police.
Everyone who has ever driven a car has sped at one point. Perhaps they did it on purpose, perhaps it slowly crept up on them on a long drive. Having a reminder to slow down is a great thing.
And for what it's worth, I spoke to a police department about Waze and asked if it's a program they dislike, and they unanimously said they love it, because even if someone reports them on the app, they get the desired effect. People slow down. So it literally does part of their job for them. They said it's a fantastic tool for them and the public at large.