Same thing here (but with Ocaml). You come into this intro course as someone who can already program and it feels like such a weird take that they deliberately steer you away from playing around and building something cool. IO being important is a hill I'm willing to die on, at least for certain kinds of learners.
Steering me away from bad habits or towards more computer science did make sense - but, yes, actively discouraging me from building things I found cool didn't help my motivation.
It didn't deter everybody, though - my friend went full speed ahead and implemented a graphical MacOS application using SML that forwarded e-mails over SMS so he could read them on his phone.
