Apple did for the original iPhone and all versions of OS X. All movie studios do. You might want to fix that generalization.
Oh, most PC manufacturers show off their new computers before they're released. As do some camera and television manufacturers. Need I go on?
Although you're right in that his generalization is flawed, your logic is flawed as well. The original iPhone and all versions of OS X have been available for preorder online, so even though you couldn't get your hands on it, you could secure an order. This simply is not the case with the iPhone 3G. You have to go to a store to get it, and you can't do that until July 11. You can't secure an order yet, so they're not going to start advertising yet.