i have read the law. the law only mentions "gross errors", not policy. i bet standing at a register with something in hand and them not agreeing to sell it to you for the scanned price is covered.
http://www.mass.gov/ago/docs/regulations/940-cmr-3-00.pdf
it is covered in section 3.13.
the law also requires them to fill out forms when this happens, which i doubt they do.
in the end, it is a stupid situation they put themselves in. i understand how they got here: laziness. they wanted a way to mark things for removal. rather than create a new status, they decided to just mark it a penny. then the MET team could scan the system for penny items. the laziness continues in not setting up the scan system to understand that penny items are not for sale, and not to show them when a bar code is scanned.
they are a large organization. this is the stuff of class action lawsuits, and it has been going on for years. it shows a complete contempt for their customers, that they would take something away from you in the middle of the store, and argue with you about why they won't sell you something for the scanned price.