One of the reasons legal immunity is being offered is because corporations are suspicious that the data will get used to enforce regulations, bring lawsuits or otherwise be a tool of legal pressure.

The Government in this case is willing to make a promise (who knows how well it will be kept) not to use the information for parallel construction or explicit enforcement. In turn they get to protect the business and American industries from cyber attacks and give the data for analysis and processing for whatever national security purposes the DoD deems necessary.

There are industries that are already onboarded - financial and energy. CISA would expand these to other industries.