This may sound like a dumb question (and it probably is ;D ), but why is that?
Not dumb at all.
Molds are made with two halves, right? Well, if you want to get the leg decently cast, they usually go from left to right. If the foot were part of the leg, then the mold would have to go in between the toes. Because of that, when the two halves of the mold are pulled apart, the mold itself may rip, thus making a bad cast in the future. To prevent tears we can separate the feet from the leg.
I actually had to do this for the Centurion Omni. It was originally going to have one piece legs, but that had to be changed for molding purposes.