This is their standard marketing decision. I guess perhaps they don't want to lower the price of their products to show they are still premium and worth the money. But yeah, that's the reason for a rumor site like this, and all this crazy speculation about new products.
From my perspective, I think it'd be much better if they lowered prices at a rate of let's say 20% per year. That way the bottom range iMac would cost here in the UK I think £800 instead of £1000. They don't need to do this through the apple store, but could be through online retailers such as amazon.
My opinion is that this would attract more buyers and would smooth out their sales. At the moment, when they release something they sell like crazy, but after a few months very few people buy. And this situation will get even worse with the popularity of sites like this growing.
To be fair, it is pretty annoying that once you buy an apple product you're pretty much locked into this waiting game. I would never upgrade my current macbook for an old model, I'd always wait until something new comes out. The same with all their other products. In a way it's like very bad customer service. If you have a pc you can upgrade any day you want, there's always something new, and most brands update their models or reduce prices often.