Worst case they can give it to workers at the end of the shift.
You can order whatever you want unless a payment is made and product accepted a legal transaction hasn't occured. Missed items, uncooked food, poor quality are common reasons not to accept and send back. You can decide to decline the transaction at any point.
What businesses do is make the person pay first. Rarely does a fastfood worker take my order.. stop.. makes my food and then asks me to pay. They take my money serve the next person, I wait to get called.
A drive through is different and they get very low rates of drivers ordering only to drive away.
Businesses have found a way to deal with this issue.
It may or may not be a crime depending on intent, but a transaction is not the issue. It is breach of contract. Laws obviously vary around the world, but the US Uniform Commercial Code is probably typical. The restaurant has made an offer. The customer has accepted it. There is agreed-to "consideration" (value) to be provided by both sides. If the restaurant does what they said they would do you, having accepted the "offer" by making an order, are bound to do what you said you would do (pay the price that you knew was the price when you accepted the offer.)
Just increase the dollar amounts by a large enough factor, and you'll see it. A company sells widgets. I order a million dollars worth, items that can't simply be put back in stock and resold to others. They make them, but I don't pick them up. I'm not arguing that they didn't do their part by making defective items. I'm (probably) not required to pick them up, but I am required to pay what I agreed. You are saying "no transaction occurred", but that's not at issue. The company will (rightly) sue me for breach of contract.
It's the same for fast food except that the static friction of legal costs vastly overpowers the scale of recovery from the breach of contract for each instance of no-show. But if a single order for $10,000 (catering a company picnic) is not picked up, you get past the friction, and the restaurant will sue for breach of contract.
If someone is selling 1 million dollars worth of widgets and the buyer is unknown prepayment is required. If known and a valid purchase order is signed the customer usually has net30/60 to pay if they don't there might be a lawsuit or a black mark on their credit file with collectors calling.
Does it ever make sense to pay hundreds to win that 2 dollars back? Your employees waste more when meals don't turn out perfect. The cost of food to the restaurant includes the wasted food (usually from employee mistakes) and those are reflected in the prices.
Whether you choose to avail yourself of such policies or not (including even getting the contract in writing) has no relevance to the legal question of whether a contract was breached.
I hadn't realized this until I read your post, but most (not all, but most) drive-throughs that I've been to recently seem to be engineered to prevent people from ordering and then driving away.
"Engineered" like civil engineering, not social engineering. They feature a single lane with curbs on either side, typically near the edge of the property (so frequently near a fence or wall, etc).
I suppose the person could order, pull through, and then drive past the payment window and keep going, but other than that it's definitely not easy to leave that single, bounded lane once you're in it.
The problem is likely that distinguishing these for prosecution/conviction, but that's probably a similarity with wage theft, not a dissimilarity.
Perhaps ironically, people frequently place orders at restaurant and then decline to come to pick them up & pay. They restaurant still ends up providing the service they promise, and the customer declines to make the payment they promise.
It's terrible, but my understanding is it's similarly quite commonplace and hardly ever prosecuted.