tracking customers/users (Score 1)

Tracking can hurt you, even if you don't do anything illegal; your viewing and shopping habits can be used against you. e.g. if you buy a lot of beer, that could be brought up as evidence in court. Employers and insurance companies would also be interested in "what kind of a person you are."

See (http://www.msnbc.com/local/KNSD/119513.asp) for an example of what grocery club cards can to do you.
Avoid trading convenience/pocket change for privacy!