They can't force you into IUP but what they can do, is say this unit is allocated to IUP members and we are putting it back in stock and point you to the line to wait to get the regular inventory.
I'm guessing (and hoping) Apple is just saving face with IUP members but telling stores just to sell it.
The article that spawned this thread notwithstanding, Apple could do that, but it does not appear that they have/are. I and many others reserved, and paid with the alternate method of our choosing, and Apple still did the sale transaction.