You're being naive. A retailer has no need to withhold sales numbers info from the manufacturer of a product that manufacturer provides them. Realtime sale through numbers provides the manufacturer/distributors with your realtime supply needs. Certain companies like Coke and Wonderbread even go so far as manage the retailers inventory by stocking the shelves of grocery stores themselves. There is no reason for a retailer to obscure sale through number from a manufacturer/distributor. The problem probably lies that every retailer/distributor is not going to have a system sophiscated enough to generate realtime data to account for every 360 sold through US retail.
You believe that MS is believing NPD data on NPD's word. The first thing MS, Sony and Nintendo probably did was check NPD's data against their own numbers to ensure validity. Given thats NPD is an independent third party and its numbers are seen as reliable, there is no reason for MS to release monthly numbers. Why waste the effort of reporting numbers that someone else does reliably at no cost to you?
Prophecy2k already brought up the point that I was trying to make, but let me emphasize some more points:
1) While in most cases a retailer has no point holding sales number from a supplier (letting your supplier knows when you are about to go out of stock actually benefits you because it helps smoothing out the supply chain), some might want to keep some information to themselves. This can be used as a leverage point when negotiating shelf budget and in store advertising budget (if I know that my console flies off the shelf anyway, why would I bother paying for appearing in your weekly in-store brochure?). This is a minor point, but one to consider nonetheless.
2) But lets ignore the first point. If a retailer is willing to provide the supplier with all the information they want, there are still security measures to consider. Do I really want Microsoft to have free FULL access to my stock database? If it is not secure, then they can freely pull valuable competitive information from it.
So this is basically a question of not only creating such a database, but how much money and effort will it cost me to link my supplier with LIMITED access to my database AND at the same time make sure that it is secure and they can't pull out whatever they want.
On the other hand, what exactly do I have to gain from such a move which will obviously cost me money and time? These are things that every retailer needs to consider
3) I don't believe that suppliers can manage retailers inventory anymore, especially after retailers realized that shelf space and in store promotions are worth a lot of money. Once retailers started treating shelf space as a source of profit, many of them simply don't let the suppliers dictate what goes where in their store anymore. There might be some exceptions with companies that have near monopoly in their business (like Gillette), but this is something that is gradually fading away and the bigger the retailer is the more power it has against the supplier (there are a very few suppliers that can allow themselves to play a game of chicken with a giant like Walmart for example).
4) One more point which I didn't bring up previously is that Microsoft is probably getting payed for each unit that is shipped to retailer (unless they have some kind of back-order agreement). When they provide information on units sold/shipped, they do it for their investors who are interested in how much money Microsoft made and that's why it doesn't matter as much if these units reached the customers or not.
NPD on the other hands is trying to give an indication of the retail business health in the US, and that's where "sold to customers" matters more. But it does give you bragging rights when these numbers show that the actual customer is buying the stuff that you push through the retail chain and you don't just stock up stuff.
But that's just another point worth bringing up, and doesn't take away from the fact that you can't get real time information from every retailer even if you cared about it.