That isn’t actually true. The price on the shelf is considered to be an invitation to treat. By taking the item to the checkout you are offering to buy it which they can reject. In practice they will sell it to you for the price on the shelf but this is not the law

I know this is a joke but was there not a court case somewhere in the US about saying in Minecraft after saying incriminating things?

Elan.school is a webcomic of the author’s experience at Elan. Well worth a read and the author is currently in the process of making it into a physical comic