The 3DS actually has a somewhat complicated architecture - My understanding is that it has a dual-core CPU, but one core is dedicated to running the OS kernel, and the other is used for games. Thus, there is also a 'kernel-mode' 'user-mode' split in a way, because the core dedicated to running games doesn't have full access to everything, and doesn't have full access to mess with the other core. You can kinda see this in action when you note that the 3DS can only ever have one game running at a time, but certain home-screen applications can be run while a regular game is suspended - Due to the application running off of the OS kernel's core and not the game's core.
This also means that an exploit in a game doesn't automatically result in full-system control - A separate exploit in the kernel is necessary to gain full control over the system. This is completely different from the Wii's setup as you noted - I'm not sure if the Wii had the OS running while games did or not, but regardless since games ran with full privileges a single exploit in a game resulted in full-system control.
Being able to easily copy and run games is an entirely fair point, but I felt it was separate from what I (And who I responded to) was getting at. An open-hardware machine running steam still has DRM on the games - And it's a bit of a given that most devs probably wouldn't want their games on platforms that didn't at-least try to have this (Though there are notable exceptions). Having open hardware, and having open software are really two different things, and it's a distinction that's only really come apparent in the later gen systems.
In the past, game systems were fairly close to being basically embedded systems, with the games running on the bare metal, so generally speaking having open-hardware in such a situation makes piracy much easier, because the only protections are in the hardware itself. Current gen systems actually have full OS's and a kernel-mode user-mode split, like a regular PC. Even if the hardware was open, without access to kernel-mode while the OS is running piracy isn't possible. I will concede that opening the hardware possibly creates an attack vector for the kernel though, so from that perspective it really isn't something that they want to have to deal with - And they don't gain anything from allowing it.
That said, if you look at Steam, AFAIK they have no real big issues with piracy. Why that's the case is debatable, but you can obviously make it work without having to lock-down the hardware.