פירוש על עבודה זרה 68:14

The Talmud first attempts to explain this with logic. When it comes to the animals offered as idols, the owner would want them fatter—more food for the gods. Therefore, there dung is something he wanted and it has value. But when it comes to an ox sentenced to be stoned, the owner certainly does not care that they be fattened. Therefore, this dung is unwanted and has no value.