Re: Provided you redefine the meaning of tied.
Clarification. Kindle is tied to Amazon in as much as if you want to buy e-books, almost all of which are DRM-protected - O'Reilly offerings being a very notable exception - you really have to buy from Amazon.
With the Sony, say, you can buy from a variety of different bookshops.
You are still tied by DRM, though. If you buy a second e-reader, and it's not a Sony, you still have to make sure your new reader handles Adobe DRM. Fortunately, many do.
Reg readers may be able to strip Amazon-sold e-books of DRM, but lots of folk can't - or won't for fear of it requiring what they may perceive as dodgy software off the net. So, if they buy into Kindle, they're stuck with Kindle to view ebooks purchased from Amazon.
Now, Amazon at least supports iOS, Android and other platforms so its e-books can be read on other devices. That may be true of DRM'd ePub e-books too, but it's a while since I checked - Adobe Digital Editions software p**sed me off so much, I vowed never to use it again.
Does tie-in matter? No one here likes it, me included, but Amazon is not going to go titsup anytime soon, so you can argue your purchases are safe. If an ePub seller goes under, you can still read their offerings on any ePub DRM-supporting device.
The only barrier is the ePub-to-Kindle, and that is surmountable with third-party software if you need it. So, more a hassle than a barrier.
So, none of these devices are truly open, but they are openable. But the hassles are the fault of publisher-imposed DRM, not Amazon, Sony or Kobo.
Heck, even Apple dropped DRM from music the first chance it got.