You CAN get a commission, however, the purpose of the school is (as stated in the Mission, which you will learn verbatim during indoc) to graduate Merchant Marine officers. Officers of the merchant marine are basically civilians who operate (as deck officers or engineers) cargo ships, tankers, tugs, and all manner of commercial vessels. Upon graduating from KP you will have a US Coast Guard 3rd Mates license or a 3rd Assistant Engineers license. From speaking with graduates It seems that the license is much more valuable than any active duty commission, and after you sail on it for a few years it will really take you places.
However, some say that jobs in the maritime industry have not been plentiful and I believe as much as 1/3 of 2009 grads took active commissions.
You can also apply for a waiver of sorts to work as a civilian in a shore side maritime-related job (shipyard, logistics, etc).