It is not stupid, it is removed. Tree roots, with time, will dig through/break concrete. Unless the trees are heavily hindered from the get go. Either way it will lead to leaks unless heavily controlled since even small roots with enough time can damage the building structure if allowed. This is a well known issue and not one you want 20 stories up at this scale.
The soil will lose nutrients, so unless everyone there how to compost constantly, then fertilisers will be needed to keep them alive, do you want that in your balcony?
The whole notion of doing this is outright irresponsible from an architecture and lifestyle perspective outside saying, "Look it has green on it, and hoping people are stupid enough to fall for it.