Firstly there is the emotional response: I don't want to be followed around in everything that I do for someone else's benefit, and I not at all convinced of arguments that targetted advertising is done for my benefit.
Then there is the fact that a large amount of data about me is being stored, possibly insecurely for people with even less scruples to analyse. I have very little to hide (white, middle class, straight, male, cis, no criminal activity beyond some unlicensed TV/film access, etc – there is little or nothing about me that would be frightening for anyone else to know) but there are many out there who do have things that could be (unfairly) held against them with terrible consequences. Consider women in Texas where there is effectively a reward/bounty program to encourage snitching on those who have had, or are considering, an abortion, or people in law enforcement who don't want certain groups to be able to derive their home address with any accuracy, people in one or more closets through fear of being ostracised from their family/community and left pennyless & without support, and so forth. I grew up with friends who were gay when it was still effectively illegal to be, despite what the Sexual Offences Act (1967) said, and when getting beaten up for being gay was almost acceptable (“act more straight, and it wouldn't have happened”: something a friend was once told by a policeman that saw no cause for arrest) – the fear of consequences from collected information “getting out” and/or being used to derive other information (true or otherwise) is real and for many people not at all irrational.
Back to my icky feelings, which are perhaps a little bit less rational: I wouldn't be happy with someone following me between shops, watching what I'm perusing, then to the pub and noting who I was there with, then back to my home, in order to be able to serve me relevant ads (perhaps for shoes that would be more comfortable for that much walking? or for condoms because they noticed I was accompanied by a female friend, and you never know, right, nudge nudge wink wink), and I'm not happy about the same happening in a more virtual environment. How do I trust that is really (or only) why I'm being followed? And I how do I know who else my stalker is selling news of my activity to?
[actually, the “I have little or nothing to fear” isn't entirely right – any of us could suffer from plain old identity theft in various ways]
Then there is the fact that a large amount of data about me is being stored, possibly insecurely for people with even less scruples to analyse. I have very little to hide (white, middle class, straight, male, cis, no criminal activity beyond some unlicensed TV/film access, etc – there is little or nothing about me that would be frightening for anyone else to know) but there are many out there who do have things that could be (unfairly) held against them with terrible consequences. Consider women in Texas where there is effectively a reward/bounty program to encourage snitching on those who have had, or are considering, an abortion, or people in law enforcement who don't want certain groups to be able to derive their home address with any accuracy, people in one or more closets through fear of being ostracised from their family/community and left pennyless & without support, and so forth. I grew up with friends who were gay when it was still effectively illegal to be, despite what the Sexual Offences Act (1967) said, and when getting beaten up for being gay was almost acceptable (“act more straight, and it wouldn't have happened”: something a friend was once told by a policeman that saw no cause for arrest) – the fear of consequences from collected information “getting out” and/or being used to derive other information (true or otherwise) is real and for many people not at all irrational.
Back to my icky feelings, which are perhaps a little bit less rational: I wouldn't be happy with someone following me between shops, watching what I'm perusing, then to the pub and noting who I was there with, then back to my home, in order to be able to serve me relevant ads (perhaps for shoes that would be more comfortable for that much walking? or for condoms because they noticed I was accompanied by a female friend, and you never know, right, nudge nudge wink wink), and I'm not happy about the same happening in a more virtual environment. How do I trust that is really (or only) why I'm being followed? And I how do I know who else my stalker is selling news of my activity to?
[actually, the “I have little or nothing to fear” isn't entirely right – any of us could suffer from plain old identity theft in various ways]