I drew the analogy that it's like the term "deadly weapon". Fists can be deadly, but they are not called deadly weapons. Hitting someone in the head with your fist is not guaranteed to kill them. Likewise you can drop a shipping container on someone -- and I'm sure this would earn you a life sentence -- but Winguardium Leviosa is not itself a deadly (Unforgivable) spell, as an arbitrary cast of the spell is not guaranteed to kill.

It's still a bit arbitrary. To my knowledge, using a love potion is not Unforgivable -- though it's clearly magical coercion and serves only such a purpose as that.