Think about this, if they have your money your sale is OVER they already mad e that sale. Now if a guy walks in and wants a 6K camera and is willing to buy it NOW....well then they will sell them YOUR camera and just keep you waiting a little longer. Why not you already paid. This way they make an additional sale on a hot product.
Also if my store jerks me around and I see others have already received the camera and I have a choice of getting one today form another store I will, because no on has my money.
Lastly they collect 6k in interest for a month or two for free. Now times that bu 1000 buyers, that's 600K in free interest for 60 days. Not bad free money I would say...even with interests at near nothing percent.
I'm pretty sure this is only happening for orders placed with a debit card (which should be avoided. you are better off using a credit card, if you have one.).
A "hold" is placed on your credit card for something like this (when you rent a car, book a hotel room, etc.), but a "hold" isn't a transaction (just a way to make it apparent to other vendors that you are nearer your credit limit).
There is no credit involved with the typical debit card, so a "hold" can't be used. There must be a debit.
As for the value of the interest, I think you have over-estimated. If they have 1,000 debit card using pre-orders (seems high, doesn't it?), that is 1,000 x $6,000.00 or $6 million they hold for an average of 90 days (using your estimates).
The interest rates on business working capital accounts (would have to be swept to something interest bearing) are pretty tiny. On $6 million, they might yield $10,000. Not enough to do it for the money, if doing so angered more than one or two customers.