It seems somehow logic....

if your recall is the computer system, then the car might not be able to recognize the old electronic chip in your car key, thus it has to be re-programmed. If the dealer failed to do that, they might have to order a new key or learn how to re-program. To replace a key is expensive for up to $200.

If they did not charge you anything, above might be the logic explaination. However, the horn should work or they have to make it work.