Ever notice that everything for the "public good" always ends with more power for the government and more capability for their intrusion into the lives of citizens?
It has been obvious to me for years. I'm sure that many lefties have good intentions, but their biggest fault is that they naĂŻvely believe that the government actually cares about us, and is the solution to all problems. They (along with many on the other side of the aisle, unfortunately) don't realize just how corrupt the government is. And they want to give MORE power to a corrupt government.
As you said, it's always for "the public good" or "security" or other great sounding things like that. But those are always pretexts for power grabs. I guess it takes some people longer to catch on.