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Epub is an open standard. However, if someone like Kobo sells an epub, the publisher often insists on DRM. DRM is not a single open standard:
Adobe has most of the epub DRM market (and charges the bookseller to use it). Amazon has a separate DRM for its books. I suppose that Amazon could apply its DRM to epubs but there isn't any reason for them to do so (from their point of view). (Actually, most Amazon ebooks these days are fairly similar to epubs, I gather)
The various DRM regimes each require a reader than can decode that DRM. The Amazon Kindles decode Amazon DRM but not Adobe (or Kobo kepub). The kepub reader can not decode either Amazon or Adobe but it can read the DRM Kobo applies to kepubs.
Of course, the participants in this discussion usually strip DRM and so, for them, a epub is an epub, regardless of who sold it to them and--for them--multiple reader programs make little sense.
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