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).
>>alkona+(OP)
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.