It says right on the purchase page for everything in the iBookstore that you need an iOS device to read it. One exception is books from O'Reilly. And of course apps that are not from the iBookstore as such.
To tell Apple you don't like this, you can use the feedback channels.
In the meantime, if you want to read purchased ebooks on OS X, you need to get them from a different source, like Kindle, Nook, Kobo, or Googlebooks and use the free ereader apps available.