You're overthinking this. There's a publicity element to it, but the money just gets given to charity, like they say it does. It's not a conspiracy or a tax accounting trick.
Same way I can tell that my next door neighbor isn't poisoning my water supply or that Tesco doesn't have a secret chemical weapons program. That major supermarket chains are running scam charity appeals just isn't a hypothesis worth entertaining in the absence of any evidence for it.