And based on Apple success the population WANT app stores to buy their stuff at so MS adding one is just giving the majority what they appear to want.
As someone who is neither ignorant, nor lazy, I download and purchase all my tablet software through the official app store (Google Play) for my device. nVidia has an app store. Lenovo has an app store. Amazon has an app store, <Tablet Vendor X has an app store> etc... I don't care. There are nice games, books, and other things exclusive to other app stores. I won't buy them. I even nuked the alternate app stores on tablets I gave away as gifts, because they are a pain in the ass.
The only difference between app store A and B is who gets the 20%. So there is no need to support 50 of them as a user.
The Steam thing is kind of funny. Because Steam itself is a unified vendor in the same vein. It offers the same pros and cons. Users have a nice unified place to purchase all their software. They have one account, one collection, and one place to check for updates and etc... We tablet users want that on our platforms of choice too!
It's nice to turn on the tablet, hit one button, and have all my stuff managed and updated as a collection. It's nice to open my Google Play bookshelf, and see all my books there as one collection, and not a ton spread across different readers (kobo, kindle, etc...). If a book I want is not available on Google Play, then I'll get a different one that is.
Tablets do not replace desktops. They are mostly simple consumption devices. You pick it up, sit on the couch, and read a book, flip through a comic, play a game, listen to a podcast, watch a youtube video, look at your facebook, browse the web, etc... All consumption type activities with minimal input needed.
So there is no "freedom" to be taken away.
It's also nice that having a tablet makes for a nice companion to move the consumption activities off the PC, instead of multitasking all these things on one device. Let the PC focus totally on your creation and computation activities, while the tablet has the news or youtube or whatever other consumption activity off to the side.
Surface doesn't change this. Sure they include a physical keyboard, but so do countless other tablets. You can even buy surface like keyboards for your android or iOS device already. It doesn't change anything. It's about as useful as plugging in a keyboard to an PS3. It helps when entering text, but that's a rare activity. My tablet even has an optional keyboard dock that turns it into a netbook. Not buying it was 100$ well saved ;).