Well, the sudden appearance of the blue post certainly changes things.

PRETTY CONVENIENT, SACCO!!!

Okay, I kid. But seriously...

1. It goes without saying that they have to add a lot of servers before Cataclysm goes live, and with all that's on their plate with regard to getting the new content ship-ready, the possibility that they got all the new servers installed six weeks early is a pretty hard sell. How's this relevant? Because...

2. If they're making digital purchase available on release day for the first time ever, they're going to want to make sure it goes off without a hitch. Hence why, as you stated, "you can start downloading the remainder of the expansion's data THE MOMENT YOUR PURCHASE GOES THROUGH" (emphasis mine). The requirement for payment is the filter which makes sure these downloads start happening in a more steady and predictable flow between now and release day, rather than all at once.

For them to just decide, out of the blue, "screw it, let's let all 12 million of them know exactly how to start their downloads tonight", an awful lot of things would have to be ready (including having been stress-tested) that I have a hard time believing actually are.

There's a strong possibility a certain blue poster is getting a stern talking-to before the afternoon's out.