For the same reason that sending me a postcard doesn't entitle you to enter my house and take an inventory of my fridge. To get that information about a user, you should need informed consent, not just a marketer's wistful greed. That same "why not?" attitude is exactly the same bullshit user-contemptuous line of thinking that the NSA and the FBI use with their "intercept it all and let God sort it out" eavesdropping schemes.