Spin versus content
"... to allow government agencies to share information on new security threats and attack vectors with private companies" -- They surely can do that already, after all it's the job of the police to keep everybody safe.
"companies can choose to share information about their users with the government" -- There is a well-established framework for the police to get somebodies data. It usually involves reasonable cause for suspicion and judicial oversight.
So the thing the police really wants is a short-cut to remove judicial oversight. And the companies would love legal immunity. So it's a win-win situation if you ignore that it destroys a century of well-designed legal precedent.
What would <insert tyrant of your choice> do in this situation?
What would <insert respected democrat of your choice, e.g., Churchill, Roosevelt, Mandela> do?