Major phone networks express concern over video sharing site and parent company’s inability to guarantee money won’t flow to extremism. The growing advertiser boycott of YouTube has reached the US, with AT&T, Johnson & Johnson, Verizon and Enterprise all halting adverts on the site, as well as Google’s wider ad networks, over the video-sharing site’s inability to guarantee promotional messages won’t appear alongside extremist content. Some of the new members of the boycott have also expanded the scope of the complaints, from a specific focus on YouTube to a broader criticism of Google’s ad platform. “We are deeply concerned that our ads may have appeared alongside YouTube content promoting terrorism and hate,” AT&T said. “Until Google can ensure this won’t happen again, we are removing our…