Some very strong words. I don't know if i'd go as far as saying it is the "holy grail", however, I do see your point. For sure it allows propaganda, misinformation, and lies to be disseminated at a very fast pace.

What would the reasons for government to ban it ? (public and behind the scene reasons)

"Continuing Transparency on Russian Activity" https://news.ycombinator.com/item?id=15758852 "It is important that people understand how foreign actors tried to sow division and mistrust using Facebook before and after the 2016 US election."

FB angered the US government by allowing foreign agents to use its tools. So that's only an example of Russians using it on Americans. Now imagine how the US government is using it. You are not going to see it in this "Transparency" report that's for sure.

Or imagine corporations using it.

Yes, Monsanto/agribusinesses/GMO or Pharma/Opioids crisis possibly are using FB to sway public's opinion. It would be great if this FB's transparency tool wouldn't be limited to Russians. I think there is an opportunity for development of some sort of network analysis tools or visualizations similar to how newspapers analyzed Panama papers.

Public: Russian election hacking

Private: If we can't co-opt it then it's not useful to us