Electronic payments are an entirely separate issue and there ought to be strong legal requirements preventing retailers and credit card companies from sharing data about people's purchases. Or, better yet, anonymous electronic cash like BitCoin.
While the invasiveness of a database of purchases linked to identities by credit card is substantial, it is orthogonal to the collection of how the user walked through the store. That hardly adds any additional value for anyone when it's just one datapoint. It's completely useless to insurance companies, maybe useful in aggregate to other stores. The point is to more effectively lay out displays to drive impulse purchases from people in aggregate. It would not be in Target's interest to let Wal-Mart know what the traffic pattern is in a Target.
If you are not already disclosing the fact that you shopped at the store by using a credit card, then there is nothing to tie your MAC address to your identity.
The MAC address doesn't lead to your name, address, credit card, or anything. It's just how they tell one phone from another.