The purpose may not be to save money. The purpose may be to have flexibility. To install only what the user wants to install. Maybe the user does not want OSX preinstalled on the HDD. Maybe the user wants to boot diskless with a blank HDD only used for storage or something else, but not the OS.* The point is that it allows more flexibility. Let the user decide what she wants to install. Sell the user the hardware. Charge whatever you want. Then stay out of the user's business. How she uses the hardware is her business.

*There are some real security benefits from keeping the OS on read-only media.