The whole point is that when you're collecting data from members of the public, they don't give you their personal data to do whatever the heck you want with it. They give it to you for a particular purpose. They have to pay their taxes. They're required to do that. They realize that. They're law-abiding citizens. They give you the information necessary, but that doesn't mean that you, as the government, can do whatever the heck you want with it. They give it to you for a particular purpose. It's called purpose specification. It's use limitation. You are required to limit your use of the information to the purpose identified. That's fundamental to privacy and data protection. Personally identifiable data, which has sensitivity associated with it, must be used for the purposes intended.
Michael mentioned the Stats Canada debacle when they wanted to collect everyone's financial data from the banks. Are you kidding me? I'm sure you know how much outrage that created. They wanted to collect this from 500,000 households. Multiply that times four. It's completely unacceptable.
You have to be very clear what you want to do with the data and obtain consent for that legitimate purpose.