Call it whatever you want, the way big companies like Amazon operate has moral implications and, therefore, Amazon has a moral. The fact that is isn't a single living organism but a sum of many doesn't mean it has no morals whatsoever.
I do understand your line of thinking - how do we create a law that can't be exploited? Because, after all, we expect it to be exploited at some point.
Although in my country the rules are the same (the lowest announced price has to be honored) there was a issue a few years ago that a product was announced at a much lower price. A judge ruled in favor of the seller. I don't really recall the details but I think the ruling was based on the fact that the law was supposed to avoid sellers exploiting buyers, but the announced price was a clear mistake.
Common sense above all, I guess.