Yes, it assumes some knowledge of the language. But imagine if every article about, say, Ruby, had to explain things in a way that someone who'd only ever seen C would understand? Authors would be hamstrung, unable to talk about things at any level other than the most elementary.

If you're going to talk about the power of a language you have to allow the use of language that talks about that power. To talk about what you can do in Haskell beyond toy problems you need to allow people to use more than just toy language.

If readers aren't willing to do any work, its tough to help them learn anything new.