That wasn't true when I competed. I actually picked up a complimentary membership because I competed, if I remember correctly.
My institution was ACM-affiliated, but I'm not sure that matters either.
My institution was ACM-affiliated, but I'm not sure that matters either.
And my institution is ACM affiliated as well, and also paid for my entrance fee to the ACM. But now that I think about it, I believe you're right, I don't think the ICPC required it.