I wouldn't ban them, but with those I also have a pretty clear view of exactly what information is trading hands. I tell the store exactly what products I bought , in which store and at what time. And in return I might get a rebate or similar. In that situation I can choose NOT to show the loyalty card when the kickback isn't good enough or if I'm buying something I don't think should go in my "profile". (And for this reason, I hope stores wouldn't link these purchases using any other means I don't approve of, such as through the same credit card being used).

But the data could still be misused, even with your consent. If that's the bar, it's extremely high, given almost all data can be misused.