I am intrigued. I have been to restaurants before where individuals brought in their own special occasion cakes, and I've never seen a charge applied. So I can understand why the teens were a bit put-out on it, especially since birthdays and showers usually mean that people relax a little more and spend a little more than normal (at least the people that I know do!).
I understand why the teens questioned it to, especially if they didn't understand it - there's nothing wrong with inquiring as to why you're being charged something. If the teens became belligerent, that would be another story, but it seems as though they eventually decided to just put the cake away and serve it elsewhere.
I understand why a restaurant would have such a policy in place, and I would respect it, but I just don't know that I (personally) agree with it.