Our opinion is the following. Our challenge checkers are contributed by the community. In other words, we are not creating the code ourselves. Therefore, we can not guarantee its correctness, if the rules of the challenge has been correctly interpreted, if the rules has changed, or if the code is bug free.
It would be wrong for us to send emails to people that might not have fulfilled it. In my opinion, it would be fairly easy for a cache owner to run the checker on every new found log he/she gets on challenges, especially when it's doubtful. If the checker says no, then he/she should look closer at the users proof, or contact the user if there is no proof attached.
I am also a bit unsure about your idea on one point. Whom do you suggest we contact? We never send emails to anyone who hasn't actively asked for them. So we can not send to users who logged a challenge and shouldn't. In fact, we don't have any email addresses if they have not authenticated at Project-GC and added the email address, and verified it.
It could be another notifier service at Project-GC, for challenge cache owners. This we could consider if there was demand for it. But personally, I think it's just as easy to check them one by one. At least if you don't own a lot of them.
Please correct me if I misunderstood your question/suggestion.