I totally hear ya! Interesting example of your own too. I think it's laziness too. Nothing gets my goat than a lazy GM. There was an older thread that I participated last year where the question is "What are some of the no-goes?" or something along this line. The original poster was basically asking what are our pet peeves with a DM that will make us want to quit their game.
Pulling adventures out one's butt as they go along is one of those pet peeves of mine. I'm sure that there are DM's that claim that this style is awesome, they have their players filled with raw emotion, and that they will talk for months about their game, and that they have players lined up around the block to in their game, etc, etc. These DM's sold me on this too and gave me the same pitch. But the truth of the matter was that their games sucked, dragged, had no plot or a weak plot, no real villain, random encounters that didn't make sense, and endless roleplaying over haggling, buying mundane stuff, or just general nonsense.