alt Hacker News> the same sort of reason that retailers can't be held to incorrectly published prices (in the UK at least, a displayed price is an “invitation to tender”, not a contract or other promise)
The hell? Over here, the price tags are a sort of public contract, to which the seller pre-commits. The seller forgot to change the tags? That's not the buyer's problem.
Since money has not exchanged hands, you could always decide not to buy at the counter. So atleast in the countries I have been, it is not legally binding.