This is all due to fraudulent ICOs. At current time 9 out of 10 ICOs are total scam. They do a fundraising and spend nothing on anything except for roaming around and spending in useless things. And it's a good move from social sites to block those ads until we have regulations of the ICOs.
Indeed. Many ICOs nowadays are either scam, or their project doesnt prove strong enough after the launch. Therefore, its a good step taken by social media to stop promoting such ICOs and tokens therefore securing the people that visit their site. This step helps protect the investors from fraud activities and scams. Regulation of ICOs in recently the main aim of governments in the crypto sections. Security and protection of investors is must and necessary step from the scam and ponzi scheme ICOs.