Banning ads for Crypto is the third measure Facebook is taking now after having emerged from the "Russian" ad controversy.
1., Facebook said it will seek to promote more content from people you know.
2., Facebook is looking to run more local news.
3., The Crypto Ad Ban.
In the BBC, it says that Facebook's policy going in is intentionally broad. Facebook does not believe that:
"That said, there are many companies who are advertising binary options, ICOs and cryptocurrencies that are not currently operating in good faith.”
Facebook is also worried about scams in general and their platform generating undue hype for potentially junk coins. They also stress that many of the crypto products being offered are not regulated.