Now, whether you disagree with the policy or not we can (probably) all agree that it was handled a little badly. This policy should be one of those silently enacted policies. That way minors are protected and no one becomes offended. By this I mean that better management of the seating arrangements tied into the booking system would mean that the policy could be enforced but it not be obvious to any passengers. I believe that the system already records when you make a booking that it will be an unaccompanied minor, so that part is covered. The next part is the seating arrangements. If it hasn't been arranged prior to boarding then simply issue them a new ticket. As every frequent flyer will tell you getting a last minute seating change is not uncommon. In this case, a last minute change for "operational reasons" to the gentleman's seating probably wouldn't have raised an objection. If you need move them to an exit row or something so it seems like an "upgrade". Ideally this would of course occur much earlier, say before the person even tried to select a seat, by marking the seats as unavailable on the booking system.
So my view is, good policy, bad implementation.