I'm not really seeing the problem here, could anyone enlighten me why you should use B instead of A when A seems to work just fine?
On a technical level, there is no problem, it works just fine.
The problem is that an A-A cable is not standard (my not even be allowed per USB spec, but who the heck enforces that?). It's not a commonly available cable, which also isn't really a problem since the keyboard comes with one.
The A end (host) is what provides the 5V power, and the B end (client) accepts 5V power. You plug an A-A together, usually both ends would be pumping out 5V, which could cause problems (ie. one isn't turned on, and gets 5V input on an output line).
The problem is it puts a USB A-A cable in the hands of the consumer. 'People' are dumb. And that is why the USB designers came up with an A end and a B end to the cable. With an A-A cable, you could, say, plug your cell phone charger into your USB port on your computer. Ridiculous, I know, but see my 'People' are dumb comment earlier.
Basically, I'd expect a really good reason for using an A-A cable; and the likely mis-communication example I gave isn't a good reason, though a fully understandable explanation. The keyboard as shipped is technically fine. And to be honest, most people buying an XT are going to be smart enough to not do something stupid with the A-A cable. For the XTS, it's backlit (which I really want), you'll pull in a different audience, and I think you want the proper connector at that point.