What happened to “this note is legal tender for all debts, public and private"?

The key word is debts. When you want to buy something in a store, you owe money if you want it, but you have not incurred a debt. You can just not buy it. You and the seller start at an even place, trade goods/services for money, and end even. If you have a debt, you're starting the transaction at a negative place and are trying to get back to even.