First of all, it is a good decision for facebook to ban the advertising of the ICO, since many ICO projects are a fraud project, so if they are allowed to advertise, they may face legal problems.
But that is bad news for the entire cryptocurrency market, where new information is not available through facebook and a lack of access to the market.
I believe that Facebook fears legal backlash from people should most of they ICO's that they advertise be scam projects in the first place and with the help from facebook they might be actually defrauding a lot of people.