CISPA just allows companies and the government to share security techniques and info on hacking attacks against companies without getting sued by customers and shareholders because a hacker succeeded in getting company info. Without the law, companies won't admit successful hacking attacks for fear of being sued.
I suspect that those against this law(ambulance chasing law firms) are just opportunists looking to make money suing companies.
CISPA grants them immunity from lawsuits and has provided them with enough assurance that it will not affect their business in any significant way.
Senate - get off your butts and actually create a budget that has spending cuts 3x the amount of tax increases like you promised.