You are wrong. To comply with the law, a store would have to keep thousands of dollars onhand every day to cash $100 bills when people use them to buy packs of gum because, according to you, they must accept the $100 bill to make change. It would be a failure to follow the law if they could not make change. The fact that they can refuse $100 bills also allows them to refuse $5 or change.
It’s not in their best interest to do that because it is foregoing income, but they can do that.
There is a difference between not accepting a bill by policy and not being able to make change for a bill because of a lack of change.
One is systemic and the other is incidental.
If you worked at a store that had multiple cashiers, for example a supermarket, then there would be plenty of cash back in the office that could be switched out if several people paid for small purchases with a large note.
If it is a place like a convenience store, then the manager could make a run to the bank to get change, or even (in a weird situation) get change from the ATM located in any convenience store to get change needed.
But most businesses do keep more than enough cash on hand to make change for a $100 bill. They don’t run into the situation often.