It saddens me that there are people in our profession for whom implementing such a thing presents no moral or ethical dilemma.
It saddens me that most people refuse to accept that not everybody shares their morals, and that it might be them that are wrong. I'm not saying there isn't an absolute right and wrong, but I certainly don't find this as abhorrent as most people here, apparently.
I suppose a banner saying "you are being tracked" on tom of such a billboard could have quite an interesting effect. Come to think of it I've seen "Smile, you're being recorded!" in some shops.