Some people like mice. Other people like trackpads. Apple likes both as long as they come with their credit cards.
The updated iMacs offer both as options for no additional cost. During the build process, buyers are presented with the option of either the the Magic Mouse, Magic Trackpad, or, for an extra $69, they could get both. The standard Apple Mouse is also available for those not digging the whole multitouch control scheme, which just so happens to be one of the best things about OS X.
Apple clearly now puts the trackpad on the same level as the ubiquitous mouse. Mutlitouch controls are built into the core of OS X and featured in most 1st party apps. Apple debuted the Magic Trackpad last summer and is a fun and practical way of controlling a Mac bound to a desk. It features a surface area 80% larger than the trackpads found on MacBook Pros, which is a must with the large screen real estate found on the iMacs.
Still, some people like the standard mouse and so Apple is leaving it up to the buyer. There’s no reason to force a control scheme upon a person that’s buying an expensive Mac — especially when the person is opting for a higher profit margin item like a 6 year old mouse.