1. The answer is found in the Gemara in Beitzah (6b) which states that "Muchan l'Adam is not Muchan l'Klavim" because a person does not throw to the dogs something that is fit for humans. However, the Gemara there also states that "Muchan l'Klavim is Muchan l'Adam" because a person has his mind on anything which might still be useful for himself.

2. Accordingly, they would have fed scraps of meat to dogs if those scraps might have still been usable for people. Instead, they would have thrown a Tereifah or Neveilah to the dogs as the Torah says, "You shall not eat Tereifah meat, but you shall throw it to the dogs" (Shemos 22:30).

3. I would like to suggest a Chidush and say that it may even be forbidden to give the meat scraps to the dogs. This is based on the Gemara in Ta'anis (end of 20b) which quotes Rav Huna who says that one is not allowed to give to animals food which is fit for human consumption. Rashi there (DH Ein) explains that this is because of "Bizuy Ochlin" -- it is a disgrace for the food and it appears that one is rebelling against the plenty that Hash-m has created in this world. Another reason why it is forbidden is that the Torah cares about the property and money of the Jewish people, and it is a waste of resources to give to animals food which could be used for human benefit.

4. Note that the Amora who started off the Sugya is Rav Huna himself, so it could be that he is consistent with his opinion in Ta'anis (20b) and therefore maintains that Muchan l'Adam is not Muchan l'Klavim, because it is forbidden to give to animals food fit for people.

5. However, this Chidush might not be so convincing, because the Mishnah Berurah (OC 161:11) cites the Elyah Rabah who is in doubt about whether the Halachah follows Rav Huna.

Kol Tuv,

Dovid Bloom