You don't know B, all you know is that it's the same state that A was before you started. The goal wasn't to learn what B is, but we now have a particle in a new location (potentially a space ship vs. the ground) that behaves exactly like the original would have in any test you do on it. If you can do this to all the particles in a person's body with no errors, you've teleported that person, and there's no possibility of making a second copy, or forgetting to throw the original in the dumpster.
You don't know B, all you know is that it's the same state that A was before you started. The goal wasn't to learn what B is, but we now have a particle in a new location (potentially a space ship vs. the ground) that behaves exactly like the original would have in any test you do on it. If you can do this to all the particles in a person's body with no errors, you've teleported that person, and there's no possibility of making a second copy, or forgetting to throw the original in the dumpster.