From my experience, this is correct. My company buys products from hundreds of different 'boutique' builders that range from a guy in his garage to full operations. Each of these builders has set a minimum advertised price (MAP) for the products that we buy from them.
There are various consequences if we break MAP. Most of the time we're issued a slap on the wrist email asking us to correct pricing. We like to keep good relations with the people we buy from so we do so. They could choose to drop us as a dealer if we didn't oblige.
I can see the potential to ignore their plea for our advantage if they were especially reliant on our buying their product. Say we were one of their few dealers and buy product in bulk, pay ahead of time, etc. Once the product is in our hands, it is OURS, so we are free to dictate its price.
But there's no reason to do that. MAP, as the name suggests, only applies to the advertised price. These policies rarely take sales/coupon codes into consideration which are applied at checkout. So you can undercut competition without reducing the actual advertised price. Then the store just has to take into consideration what percent of their profit margin they could stand to lose in order to gain a greater volume of sales.
That said, I have no idea how the fashion market works. I feel that most designers/companies are screwed and not in the position to make demands from a store who's buy just financed their next collection or helped build their brand. A late delivery of products would definitely peeve the store and get the designer's message across, but as I said, I feel that many designers are not in the position to jeopardize their business relation with stores.
Still, I haven't seen a store go crazy with their pricing and immediately undercut everyone by a large amount. They have to think about their sales schedule since a lot of people wait to buy. Its a balancing act of profit margins and volume of sales.
and that is what I know