Second, they spent a sh*tload of money which is in extremely short supply, NOT to clean the human sewage out of the bottoms of all the stairwells, NOT to clear out the tons of paper and discarded-food trash, NOT to erase the gang tags and the filthy obscene graffiti, but to destroy a project which was popular, harmless, and beautiful. They have really idiotic priorities.
Third, the authorities expose the public to risk every day with their dirty and pooly-maintained metro stops. If I were inclined to lawsuits, I'd sue them for exposing me to E. coli.
Fourth, I don't think the Phantom Gardener was using the Convolvulus genus. More likely the Ipomoea. These are actually valuable around public sites. Because of their tolerance for poor, dry soils and their maintenance-free habit of flourishing, they are excellent vines for creating summer shade on building walls, thus keeping the building cooler and reducing heating and cooling costs.
Fifth, I don't think you an assume the planter is a sue-the-city kind of guy. To avoid liability, the bureaucrats could have insisted that he sign a "hold harmless" agreement which would protect them from lawsuit, and everybody would have been a winner.
Now everybody's a loser. Good grief, what a display of duh.
However, you can NEVER assume someone is the "I won't sue" type. There ain't no such animal. (There is the "they probably WON'T sue" type...but never say never. My dear friend is probably in the process of suing someone, potentially thanks to a lawyer...and I still can't get my head around it. She is one who I would definitely have put in the NEVER column)
I would bet that the big objection was that no union labor was involved.
How dare these serfs volunteer! All good only comes from the government and public employees unions!(Do I really need a sarcasm tag?)