Could be added, but then you have to define which extensions to allow for each user, somewhere, adding lots of works/complexity into the mix.
We have a CDR module (commercial), that provides much more functionality, including the ability to define users that are allowed to run reports on a subset of extensions. Price is $150 per server, we can send you a trial if you are interested to try (fill the trial form in www.asternic.net saying that you want to try the CDR Report tool).
Best regards,