Would NPD be okay if we got numbers one month after their subscribers? Their subscribers are businesses and analysts. I think, to them one month old data would be useless, so they have reason to keep their subscription. Rest of us would be happy with delayed data as long as it is consistently available.
I believe some hardware and software numbers become public has zero potential for motivate a subscriber not to pay for the data anymore.
NPD simply doesn't care with those who don't subscribe, even not affecting their business they don't want to disclose anything, except a ridiculous top 10 without numbers and ranked by revenue.
Another possibility is that one or more subscribers asked them to not release numbers for the general public.