The very fact that Apple will have a potential order book of 10s of millions is why they can, and do, restrict initial supplies.
No newspaper, news channel or website ever ran with a headline of "New iPad on sale, and there's millions in stock". Headlines of "New iPad SOLD OUT in minutes" is much more likely. We've all seen soundbites from grinning phone shop managers all gushing about how they sold out in minutes. They obviously can't say that they only had 5 to sell, but, they never get asked anyway.
Apple are sitting on $billions of cash reserves, they don't care if they prize $700 from little Johnny Fanboi on the first day or the 30th day that the iPad is on sale. They know that there's millions of gullible, sorry, loyal customers out there and by artificially restricting initial supplies, they will just increase pent-up demand and make the devices seem even more exclusive and desired.
You think it's all about manufacturing delays and stock costs. It's not. It's marketing, something Apple excel at.