because some little kid (since their parents either would not be there or be busy doing something else) would climb the barrier - and you better believe that there would be one and fall off.
Disney is kids first.
of course even I don't understand why the wall around the balcony on TOWL does not have a kids place to see the fireworks.
they could have put in something to make a clear area for little ones to see the fireworks - but did not.
now it is a great area for adults but for kids younger than 10 - don't think they can see over that wall?