Someone can already walk around a grocery store and see what's in my cart or they can just watch me checkout. Buying groceries isn't exactly a private experience unless you buy your stuff online. Securing tags in things like a passport or credit card is important since identity theft is real and hardware to read the tags is cheap. Knowing what things I buy in the grocery store might help some marketers know what sells but I can't really think of any nefarious use of that kind of data.

Maybe you could use it to make some guesses about a person. An older guy buying frozen dinners, beer, and not much else is probably single or divorced. A woman buying prenatal vitamins is probably pregnant. Someone buying both Special K and Fruit Loops might have a family. But someone can already make these determinations from using their eyes. RFIDs might make it a bit easier but don't really add any new infosec facet to this grocery store experience.