I guess it might be a cultural thing? The shelf price is the binding price here in practice. If you get to the register at the supermarket and the price is higher, you can just let them know and they will send someone to check the price and match it.

Yes, seems to be partly regulation and enforcement partly cultural in a voluntary code of conduct that tends towards benefit to the consumer. For example,

[All the major grocery retailers] are signatories to the voluntary code of practice for computerised checkout systems in supermarkets. Generally, this means that if an item is scanned at the checkout at a higher price than it says on the shelf or as advertised, a customer is entitled to receive the first item free and all multiples of the same item at the lower price.

https://www.choice.com.au/shopping/consumer-rights-and-advic...

This practice not just matches price (dang, you caught us out this time), but incentivises minimising errors (oops, our bad, have it for free).