It should be legal. Caveat Emptor applies always. to everything. I'd rather have choices available in a marketplace rather than a nanny state stifling innovation through its own incompetence.
With computerized registers and loyalty programs, it is way too easy to profile. I am not too worried about looking like I chug too much aspirin; it's the further out assumptions or speculations that models (like I myself once designed or otherwise) might make that I worry about, that would not require proof, and would not be documented somewhere. I mean there is also stuff "like" congestion pricing to worry about, and maybe that would work better to make people think about liberties and tech. Dunno.