Upon further reflection and observation, there's something else extremely worrying about the whole notion of partisan shopping - the extension of ownership past, well, ownership.
When I buy a bottle of seltzer at Wal Mart, I give Wal Mart my money, and they give me their seltzer. I take the receipt, walk out the door, and while I might have the right to return the unopened seltzer, my ownership of that money is gone. It's now Wal Mart's money. The company is free to do whatever it wants with it. Likewise me and the selzter. I can drink it, use it to take out a stain, shake it up and leave it as a practical joke, or pour it on the ground in their parking lot, if I feel like it. My seltzer.
Their money. Just as Wal Mart doesn't have the right to follow me around and police my seltzer-usage, I don't have the right to tell Wal Mart what to do with its retained earnings. It can spend it to clean or open stores, pay employees, put it in the bank, or even buy its executives minor-league baseball teams for their own enjoyment. The shareholders may disapprove of the last, but I can't.
This may seem pretty simple, and it is. It's called, "ownership," and it's something that the most recent Nobel Prize winner in Economics built his life's work understanding.
And again, once we establish the principle, what's to keep a government from establishing a commission - purely advisory, no doubt, no doubt - to, ah, guide corporate political and social giving? Or to establish, you know, purely voluntary guidelines for companies wishing to establish branches within city limits? I mean, after all, it'd be much more efficient than actually having to declare and collect taxes...