It's not just poor coverage that is a problem. If, like me, you live in a small country up off in the corner of the civilized world, then half the times when you've travelled far enough to actually need your GPS you will have wandered across some national border. Trying to download maps over 3G in such a situation will basically lead to a phone bill that will require you to re-mortgage your house to pay off.

Both could be solved with "preloading" the phone. So - I dare to say that he knows how to disable that and that's completely unrelated. He doesn't even want to need data roaming to have Google Maps available. You misread the intention of his post.