You can buy the firearm from there, but the firearm has to be shipped to a licensed dealer that lives in your state. You might want ask your local PD because it could vary from state to state.
One difference is that an Excise Officer only deals with possession of drugs or alcohol while an ATF officer deals with drugs, alcohol, firearms and explosives.
It's a program that is offered to teenagers that want to learn about all the different aspects of Law Enforcement and at the same time they can receive hands on experience. It can help them get in to the Law Enforcement field.