Because if you really need to be anonymous, it's not good enough to catch it being used after the fact.
Notifying the user after the fact sometimes works for things that are nuisances, but isn't good for privacy/security (imagine if your browser would execute unsandboxed JS and show you an icon each time it did it).
It wouldn't have to be after the fact. Firefox already has a little pop-up when sites want to get your location. The options are yes/no/never for this site.
Notifying the user after the fact sometimes works for things that are nuisances, but isn't good for privacy/security (imagine if your browser would execute unsandboxed JS and show you an icon each time it did it).