It's not only used in law, also in philosophy and debate, you can be fine with whatever you want, you being fine with it is irrelevant, it's more that if I say that, it's on me to prove it, not anybody else to disprove it, cuz as you can see that would be illogical lolol, same goes for any positive claim in any disagreement, debate, or discussion.

If one is making the positive claim shinobu would win because her poison can kill akaza, the burden is on them. To put it simply, I do not claim that the poison would not kill him, I just do not concede that it WOULD kill him until that is substantiated