Screw you over, how, exactly?
I'm sure you can imagine some dark patterns. Screens that show you products they want you to buy based on your patterns. Discounted offers tailored to you that are not really discounts.
Region / time of day / per-demographic brand-popularity based pricing for a start?
There are arguments about how companies will use collected data in undesirable ways.
But I wish that the stress of living in a panopticon would be argument enough.