They don't lock you in. The point of an entry-access system is to gate entry, not exit.
To get in, you need to register a credit card. The store observes what you take, and charges that credit card for those items when you leave. If you're inside, you've necessarily given them everything they need to charge you asynchronously for everything you take while you're there. You can leave at any time.
The point of entry-access is to prevent people who don't have a registered active credit card from coming into the store, and so walking away with things that the store cannot then charge to anyone's account.
