Hahaha, you people are funny, not to mention know absolutely nothing about marketing. First, marketing costs money. You don't suppose they actually saw the movie and decided to spend less money than they already did on the piece of shit, cutting their losses even more? The biggest mistake they made was green-lighting the sequel to begin with on the expectation that it would do well.
The problem is, a good majority of people who saw the first film realized what a piece of crap it was, and decided they don't need a second helping shoved down their throat. You can take my money once, but I'll be damned if I'm going to let you do it twice.
Secondly, marketing the movie more than it did wouldn't have helped a single bit. Who are you going to market the film to? Seriously, you have to be a complete moron not to realize how limited an audience a Rob Zombie film has. What is somebody to do? Put more banners on horror movie sites or get a street team to hang posters in hill-billy homeless sections of town?
Third, releasing the film in August has its upsides, as it's still the summer box-office season and attendance in the fall typically dips, EVEN FOR GOOD FILMS, because there's TOO MUCH ELSE COMPETING FOR PEOPLE'S TIMES. There's college football. There's profession football. There's the Major League Baseball playoffs and the World Series. There's the start of hockey season. School has started, so throw in all the high school sporting events people go to. Unlike some of the twelve year olds that post here, most other people have lives and aren't wrapped up in how "Fucking Awesome!!!" Michael Meyers or Rob Zombie is.
A movie such as The Sixth Sense was released in August, not to mention became the highest grossing thriller of all time. The problem here is, people seem to be looking for any and every excuse under the sun to blame why it's not doing that well, when the bottom line is it's just a horrible movie.