The non Windows based tablet market is kind of irrelevant and doesn't factor into cannibalization anyway. And yes they can, again it's called competition, if the surface didn't have to pay for the windows license, then you might have a point.
otherwise, as I said, all the OEM would be cannibals as well, which they are, but it's called competition. Cannibalization is what MS did when they released the Zune and became the second most popular PMP, while Apple didn't lose any sales and all their plays for sure partners lost out. That's not what's happening here. In fact, the surface in many ways is beneficial for the OEMs. as their devices are cheaper, whenever they decide to release some. So they benefit from MS ads and market penetration, and then they see a cheaper device that is the same thing and they go for that. They're basically paving the road for them. Something all the OEMs are capable of seeing except one, which is also not known for it's reputation of high quality hardware.
There are more types of cannibalization than just internal cannibalization. The type of cannibalization I'm referring to here is called market cannibalization since, obviously, the partners are not internal to Microsoft.
Additionally, to see Microsoft as pure competition is wrong. Competition works when all of the players are on equal footing. In the Windows RT market Dell, and other OEMs, are not on equal footing against Microsoft. At the most basic level, MS is the branding associated with the most visible part of the product, the OS, and they benefit enormously from that.
If your argument made any sense, which it doesn't, then Microsoft would ensure that the Surface was as accessible to consumers as any other OEM is going to try, but they won't do that. They are purposely keeping the Surface out of B&M and away from Amazon and it isn't because they hate the color of money.