How could it possibly be a legal problem? Here's how I see it..
Any content that is viewable only by a guest but not by a registered member should only be content that the admin put there. If an admin is using a "Guest message" hook, then that's not something that you can report anyway.
If someone (as a guest) stumbles upon something that could be a legal issue (copyright infringement), then that person either owns the rights and thus would need to send a notice (which would be done by email) or they're just wanting to be helpful and can always opt to send an email if they don't want to register. If they submit a report, certain information is going to be collected anyway (IP address for example) so no matter which way they go, they're leaving some sort of a trail.
I'm not saying that guests shouldn't be able to report content, only that if an admin wants to restrict it from guests, then that's up to them. What doesn't make sense to you might make perfect sense to someone else. As in, for your site it might not make sense, but on someone else's site, it might not make sense the other way around.
I believe that when a spam control method is chosen for guests, it should be used anywhere that the admin wants it to be used. As in, if the admin wants it shown when someone posts a message as a guest, then it shows there. If they want it to show when leaving a comment on something (blog/calendar/downloads/gallery) then it shows up in all those places. Same with reporting content. Should be able to decide when it shows up. The method used though should remain the same. As in, not one captcha method for posts, then another for leaving comments, with yet another for reporting content. All the same but only used where the admin wants it used.