Microsoft wants to use brute force to get what Apple has evolved into.
It is possible to install alternative software on the iPhone. Not easy, but possible. Most people do not because for the market Apple is going for the iPhone and iOS work great for them, well enough that it simply isn't worth the hassle of changing to iDroid. Microsoft wants to get into the ARM computing market with more powerful hardware, and then lock that hardware down to their own software so it is actually impossible to install alternate OS's. Rather than actually trying to design something better with a software library big enough that none but a small sliver have an interest in switching, they are browbeating hardware vendors so that people will not have the choice. I prefer companies that win on innovation and (though I often loathe to admit it) marketing (but definitely innovation first by a long margin, make no mistake) over lockouts.
If companies continue to produce equal tablets across the board, just that some of them are hardware locked to only ever run Windows, then there isn't really a problem: people vote with their dollars and get the tablet that runs the software they want.
My fear is that Microsoft will use this as a leverage tool, push hardware companies to release their best and fastest tech for Windows first, giving Microsoft a head start. If that happens, and Windows is actually terrible, it will take two generations of hardware to weed it out (about 3 or 4 years I think, on the assumption that most people don't buy a new tablet every year) but by then the people who jumped on the "faster/better" band wagon might already be marketplace committed, and that's how Microsoft will win.
But like I said, that's only if Microsoft, being not an actual hardware vendor themselves, convinces the tablet manufacturers to release Windows branded tablets a step faster than the similar Android tablets, and if they can get ahead of Apple in this as well. While the first might happen with enough money, I doubt they're going to beat Apple at their own game, so I am not actually worried.