Sorry, but this is completely upside down:

“Why would I buy a tablet that only worked with content from one provider?”

Nobody is buying a tablet that only works with content from one provider. Eg you can read Apple, Amazon and Adobe ebooks on an iPad. The point is: Who would buy content that only works on certain machines. And that’s a very different question.

And it works the same way in the PC business. You just don’t notice because Windows has a near monopoly. If you don’t like the direction Windows is heading, try switching to Linux and see how much of your content is still usable.

If everything has to compatible, competition is low. And if competition is strong, comparability suffers. The middle ground is usually fine. The only thing that’s universally bad for the customer is DRM.