> Only by social convention. Not enforced by the language's semantics

Many FP languages do not enforce purity (OCaml, F#, Clojure). Even in Haskell, one could use IO and IORefs everywhere, the semantics fully permit writing "Java code" in Haskell. We need a culture and an understanding of what it means to write idiomatic Haskell.

EDIT: responding to your edit:

> The problem here is the inability to do functional programming. The reasons for this are technical, not cultural.

Yes. But it's not just problems with semantics, it's also the syntax, defaults and available libraries. Culture feeds into all of this, especially as the language and ecosystem evolves. I'm arguing that it does matter.