Have you seen the pictures of those balconies? They're pretty small. It was stupid to cram so many people onto small wooden balconies. There are limits to everything. Also, were the "victims" just standing there or were they moving a lot? Dancing and jumping can put a lot more stress on the wood than just standing still.

How much weight per square foot should balconies be expected to support? It appears the balcony was fairly recent constructed and from what I could see, it looks like they used good materials. Unless the balcony was constructed below the building code standards, the blame for the accident belongs to the party-goers, most particularily those on the highest floor.