As a general rule, injections are a better, faster, more precise, etc way of delivering medications than orally. A snake's slow metabolism just makes this even more true than it is in humans.
The vet probably thought he was doing you a favor (most pet owners would prefer to give oral medications), and possibly didn't even know how to properly give injections to a snake.
I'd recommend trying to find a real herp vet and get the injectable meds.