While this is an interesting idea to encourage OEMs to promote their Win8 devices more (as long as their cut comes out of MS's 30%, not a further reduction of the developer's cut), there are some practical issues.
For one thing, I'm not sure how easy it is to verify which device an app was sold on. Sure, OEM information is often stored in the registry, but that's open to abuse. You wouldn't want an employee of an OEM to modify his regular desktop PC to falsely claim it's a device from that OEM.
The other part is that an app, once bought, can be installed on any device. I can buy something on my Windows 8 desktop PC, and install it on e.g. an Asus tablet, without Asus getting a cent that way because I didn't buy the app on their device. The only way around that is to require you to purchase the apps multiple times, once for every device you own, and that wouldn't go over very well either.