Everywhere I looked when I was last in Manhattan (in May) there were trees on the roofs of buildings and gardens.  These were not all new buildings, in fact most of them were classic buildings that managed to green up their roofs somehow.  So, all the objections seem to be a bit much - green roofs are a good thing and it makes the roof space more usable by humans.  Perhaps it is this that is really being objected to - if we make roofs nice people will demand access.