First off, I am not a lawyer (IANAL) so this is not legal advice and hell, it might even be wrong.
I recently got involved (read opened my big mouth) in a discussion about RFID in the context of providing extra security for individual financial transactions. A point was made that the security of this scheme is suspect because, after all, they make wallets that block RFID scanners. My response was that I wasn't sure this was a real threat to security or privacy, or a perceived one. I was attempting to illustrate these "RFID proof" wallets could likely be a gimmick by some huckster playing up the fears of the populace.
The response I got quoted the fourth amendment and I promptly got a little confused. I'm always a little uneasy when people mistakenly think the constitution grants them some explicit right to privacy. From my limited perspective, the point of the constitution is to limit the powers of government over the people, not to dictate what an individual can or can't do.
I would argue that it is probably unconstitutional for the government to outfit patrol cars with RFID scanners and then patrol the neighbourhood for stolen merchandise (in your house) or to set up checkpoints along the highway to search for contraband...
But is it is certainly not unconstitutional for a private business to engage in activities to determine what customers do while in their place of business. After all, it's private property, there is nothing compelling you to be there... I didn't sent my thugs to your house the grab you and force to to buy things in my store (website). You voluntarily left your home, drove to my store and gave up a great many rights to privacy by doing so.
All of this is ancillary to my original point which is... who is outfitted to track RFID tags to that level of detail right now? I recently did the math and a 64 bit tag has enough unique combinations to store 10 million unique codes for every human who has ever ever lived... EVER. I'm sorry, but I know for a fact we have a VERY hard time tracking and infinitesimally smaller number of things through our supply chain, tracking that number of unique tags is going to be a lot of work (not impossible).