Does it? For a business with a low barrier to entry like selling books? I'd like to see some examples.
The reason Amazon can have a monopsony is because it keeps prices low. It retains customers and forces publishers to go through them. If they raised prices, they'd lose their advantage, and there are plenty of competitors ready to take the market, including well funded companies (e.g. Apple and Google).
If this was what's usually called a "natural monopoly", maybe that could be argued, but book selling - and especially ebook selling - is anything but.
That said, if you look at the big 4, they largely consist of mergers of very old companies, and Amazon is really the only successful ebook seller in english at the moment, even Google seems to have problems getting far in that market. So the barriers to entry may be higher than you think.
Amazon offer a painless way of adding books to the device. Buying a book takes seconds and it magically appears on the front page of the device waiting to be read. Its one reason why book piracy didn't go as insane as music piracy, the means of adding books was easier than the device
On my ipad I have iBooks, Marvin and Kindle. The first app I open is the Kindle due to the 180+ books that already sit on Kindle's cloud. And I doubt that is going to change.
As an utter aside I do have an e-ink reader. I went from an ipad and kindle to an ipad mini and then back to the former. Reading on an e-ink device a fall asleep reading on an ipad mini the light kept me awake...
My answer is: it depends. My wife and I have well over a thousand books on Kindle, but I've got a growing library of Kobo books (from taking advantage of some impressive discount coupons) and almost everything DRM-free I get outside a major ebookstore (e.g. books bought directly from publishers or authors) goes straight into Google Books (they're the best with uploads).
Yes, having books scattered across multiple apps isn't perfect, but it isn't that big a deal, either. Amazon has a significant edge (especially with customer service), but it is nowhere near insurmountable.
If your service doesn't give easy access to drm-free ebooks in standard formats, then any barriers are caused by them and not Amazon.