I don’t know why microsoft would _really_ care if you install android, ubuntu, or whatever on a tablet. In fact, its stupid not to encourage it.
You’ve already bought one so they’ve collected their license $$$. In fact, the more people that buy them for any reason is good for them. You can smash the thing if you want. No problem – go buy another. MS won’t mind. 
The iPad is different – a complete custom product with the ipad name on it and apple profits off the hardware too. If they allowed android on it, they’d worry it would tarnish their image, if seen running unstable custom ROMs.

 Part of Windows 8 goals is increasing security and part of the reason for increasing security is to better secure the platform for things like better DRM support…  Same reason why Netflix HD app requires a locked boot loader to work.

All of which would mean a better chance for Windows 8 to get pay for content. Along with Enterprise and other business usages.

The other reason includes many simply don’t trust the ARM platform yet and want to minimize chance of piracy, etc. until at least they’ve established themselves in the market.

While ARM itself has traditionally been a made to order platform.  Most people don’t jailbreak or root and use the device as is most of the time.  This is further complicated by the fact ARM is only now getting to the point it can properly run a desktop OS.  So they also don’t want to compromise the system in any way that could effect the performance and reduce the chances of people wanting to use Windows on ARM.

This leads to a reason they’ll have in common with Apple’s closed system.  Having control means they can help ensure a better level of support and more guarantee of performance.  Reducing fragmentation and other issues.  Something as already mention is more of a concern on ARM.

Though it’s also likely because no one can complain about monopoly on ARM with so many other choices available and MS just wants to maximize their chances and reduce the effect of the presently dominant competition. 

After all, there are very few Windows users in the general consumer mobile market and market momentum is something that will be hard to over come with so many used to Android and iOS and Windows 8 still has to prove itself.

it was this way all the time of win ce/mobile. win8 only the follow up of it, why changing the philosophy? ms never did by free will

 MS is trying to avoid handing Linux the one thing that is making Linux on ARM bothersome by its absence, standardized device management.

Right now, each SoC had its own variation on a theme for managing device features and peripherals. MS is dictating a set ARM based platform using a setup pretty similar to x86 desktops that all WoA devices is required to use. This however would make it downright simple to plop in Android, Ubuntu and similar. And could in the short run provide access to more software than there will be for WoA at launch.

In other words, MS is building a new playground for themselves. And do not want other kids on the block there and play without their say so. This because said kids have a stronger reputation than MS in the neighborhood…

These bastages are so blatantly greedy they can’t see past the nose on their faces!  MS doesn’t want their OS on Android tablets?  Idiots!!!  MS has the power to wipe ignorant Android off the face of the earth and they don’t want to?  Fools!  Imbeciles!!!