Thanks for your comments. I think what freaks out people (including me) is how hidden those practices are, how intrusive they are (predictive of inner characteristics applicable outside of politics) and how unexpected they can be (who knows that their shopping purchases can be so predictive?). Any politician asked directly about these techniques would have as only justification that their competitors are using them too. I am not sure this is good enough.

The most natural parallel is to imagine a real life meeting where politicians/car dealers are given little cheat cards with a psychological report before addressing each potential voter/buyer. What would make such a practice acceptable?