And what does that have to do with everything being immutable by default? Either the language supports mutability as a first-class concept, or it doesn't. Sure, you shouldn't be using mutable global state, either in Go or in Haskell.

The bad part about Go is that it's impossible to write type-safe immutable collections. Apart from that, I don't think it encourages mutable global state anymore than any other language with first-class mutation.