I never suggested that retailers were colluding, which would be flagrantly illegal. The question is whether retailers can agree with *manufacturers* on a minimum price.
The linked article doesn't say that minimum retail price agreements are legal... just that they *can* be legal in certain circumstances. E.g., it might be legal for Fujifilm to have minimum retail price agreements (because they do not have a dominant marketshare) while being illegal for Canon or Nikon to (as the converse is true).
Of more practical consideration, the fact that, according to that article, some states like California continue to outlaw minimum retail price agreements of all kinds would inevitably result in online retailers in only those states selling large quantities of such items at lower prices -- which would in turn harm the retailers located in states that allowed minimum retail prices.
In my experience, you can regularly purchase products below the (legal everywhere in the U.S., it appears) Minimum Advertised Price if you are inclined to negotiate or shop around. Therefore, I suspect most manufacturers are using MAP agreements, not MRP agreements, in the U.S.
Here is another article addressing the issue of MAP vs. actual sale price. As the article points out, in some instances in online shops you will see the following in place of a selling price: "Add item to cart to see price." Upon adding the item to your cart you will then see it at a price lower than the MAP. This would be a contractual violation of a MRP but not of an MAP since the shopping cart is not "advertising".
https://www.sba.gov/blogs/how-minimum-advertised-pricing-impacts-your-retail-or-online-stores-marketing-efforts