A difficulty on the Rambam from the Gemara Bava kama pg 23

Furthermore, the Gemara asks on the Mishna . The Mishna brings a case that a dog carried a loaf with a burning coal in it to a stack and the stack is burnt up. The payment for the loaf is full damage, and the payment for the stack is half damage. The Gemara says the reason is that it is not one's own arrows, but the arrows of the dog, so it is half damage. The Gemara then asks, “Who pays?” Answer: the owner of the dog. It asks “Why not the owner of the coal?” Answer: he guarded his coal. So, the Gemara holds that the law is that the payment for the stack is half damage, and that is paid by the owner of the dog. Maybe the owner of the coal would also pay if he did not guard the coal. [And that if fact what Tosphot says.] But we do not know that directly from the Gemara. All we know for sure from the Gemara is the owner of the dog pays half damage for the stack. So why does the Rambam write there is not payment for the stack at all? Where did the owner of the dog go? If the Rambam wanted to say the owner of the fire also pays well, he could have written that. _____________________________________________________________________________________________________The גמרא asks on the משנה . The משנה brings a case that a dog carried a loaf with a burning coal in it to a stack and the stack is burnt up. The payment for the loaf is full damage, and the payment for the stack is half damage. The גמרא says the reason is that it is not one's own arrows, but the arrows of the dog, so it is half damage. The גמרא then asks, “Who pays?” Answer: the owner of the dog. It asks “Why not the owner of the coal?” Answer: he guarded his coal. So, the גמרא holds that the law is that the payment for the stack is half damage, and that is paid by the owner of the dog. Maybe the owner of the coal would also pay if he did not guard the coal. [And that if fact what תוספות says.] But we do not know that directly from the גמרא. All we know for sure from the גמרא is the owner of the dog pays half damage for the stack. So why does the רמב''ם write there is not payment for the stack at all? Where did the owner of the dog go? If the רמב''ם wanted to say the owner of the fire also pays well, he could have written that.