Apply that same question to every other type of police surveillance. Why is this materially different?

It both is and is not different. It's different because of the obvious riots.

But it's not different in the long: we should be aware of mass surveillance, because it can affect freedom of association (chilling), and can advance government abuse.

Citizens should always watch the government, so that we have a chance to rein them in. Its our government, not the other way around.