wilting

Is not eating Nabila a matter of preference or a matter of honor?
What is the difference? (Lakau') Is it that I am commanded not to eat the dead animal or is the dead animal the thing that cannot be eaten by me (hafza), then it is the same thing, if there is a ball that is required to pass over every wall – then every wall is required for the ball to pass over it. The only difference is what the commandment focuses on first?
If so, what is the difference between a vow that applies to a mitzvah and an oath that does not? I conflict with the commandment in the same way (the wording is different…), I say that I am not obligated to perform the mitzvah, why would the Torah allow this?