That's what I thought/assumed. Now, I've never had a problem in the past when, on a couple of occasions, the I94W didn't get pulled at checkin; I simply kept it and gave it back when I next entered the USA. But now I have no real material proof about what happened (short of taking cruise documentation with me next time I visit).
I seem to recall there's an address in the USA where you could send your I94W in the past. I might try and write them a letter, see what happens.