And now we're seeing the problem with retaining socket backwards-compatibility. Great for wanting new CPUs without buying a new mobo, but the corporate penny-pinchers can take advantage of that too.
The culprit is not backwards compatibility. Some boards support dual or single channel operation, some single channel only. In both cases they can run the same processors. I am not sure how much of this is a cost saving measure or if there's other factors in play, such as creating an artificial performance barrier between their cheaper devices and higher-end models.
In terms of cost savings I am sure in many cases they are able to get by with a cheaper design that forgoes dual channel support, and also by selling the vast majority of FP4 systems with the same motherboard they better benefit from economies of scale and a more streamlined production process. Basically they can use the same board in the same chassis and slap in any processor the OEM has designed it to handle from cheap cat core APUs on up to whatever they are comfortable with. Rather than having two distinct mainboard designs for the same chassis, like they had to do before (one for Cat core APUs, one for BD-based APU designs).
On desktop, Bristol Ridge will be a new AM4 socket, that it will share with Zen. Hopefully AMD has done the same, and made a new socket on the mobile side of things.
Well as I mentioned above the OEMs like to both reduce overall costs, but they also like it when cheaper (lower-profit) systems can't be upgraded to perform like premium models even when configured with the same processor.
I am hoping issues like this will fade over time and more laptops will support dual-channel out of the box as Zen APUs start to ship. I would bet that most AM4 desktop systems will be spared such issues in the first place.