This attitude right here is part of the misunderstanding, and part of the problem. You haven't BOUGHT the ebook. You've paid for a license, and in all likelihood, that license prevents stripping the DRM.
Funny, the button clearly says "Buy now". Not "License now" or anything else. If retailers want to make that distinction clear thats fair enough, otherwise I'm perfectly happy with the attitude you refer to.
Although I do agree that the distinction between a product with DRM and a license to use a product with DRM does not seem well distinguished. Exactly what you are obtaining should be spelled out, at least in some form of legalese somewhere that you see when you buy the product, not just in some document buried on the website you're browsing.