Perhaps they are afraid that many criminals will use them as tools for fraud or maybe in the future they will release their own crypto
Not like they are afraid, a lot of scammers were already using the ad space for fraudulent activities with so many fake ICOs being pushed out there to potential investors, some phishing being done via the advert space and so many more. All these things were already becoming a crazy thing generally for the whole space, and this development actually brought some little bit of sanity into the space as far as I am concerned, except for people who just want to see the negative sides to everything.