It’s another way of cheating the customer alright.
It gives the impression that the CPU will perform at a certain level on the stock product when it will not. It’s bordering on deceptive advertising in my opinion, even if a single stick is declared in the description / specs. This is because only a small percentage of buyers (the technical ones like us) are likely to know about one stick restricting performance and then only depending on how familiar they are with that CPU’s performance characteristics.
Only way to be free and clear of any deceptive practices would be to declare in plain sight that a second stick would be required for maximum performance. An indication of the performance gain available would be appropriate too.