[Thorentis]

Yes, but I think OP is saying that the law should protect intent. Just like with the physical mail system. It is illegal to open a letter addressed to somebody else (though, warrants can override this). But I am fine with the principle.

On the other hand, if you stick a huge banner out the front of your house, that information is fair game. Just like posting on your Twitter profile or blog. The intent was never for it to be private.

[ArnoVW]

Not a correction, but I'd like to add a small precision to that because I've seen the same argument used in other contexts.

For Twitter and Facebook I tend to agree, as there is an active intent (as you say) of publication. However, I've seen people reason in the same way with respect to licence plate or face recognition: "but the information is public".

The fact that technology now allows us to treat licenceplates or faces information globally, in very cheap way, means that a fundamental new capacity is created.

[snowwrestler]

I agree, the impact of automated scale is definitely worth considering.