Because he burned the bones of the king of Edom into lime.
Burning the bones of the dead
Amos says that the Moabites were wholly perverse, that no repentance would be hoped for, as they had added crimes to crimes, and reached the highest pitch of wickedness. He mentions one thing in particular--that they had burnt the bones of the king of Edom. Some take “bones” here for courage, as though the prophet had said, that the whole strength of Edom had been reduced to ashes: but this is a strained exposition; and its authors themselves confess that they are forced into it by necessity, when yet there is none. The comment given by the rabbins does not please them,--that the body of a certain king had been burnt, and then that the Moabites had strangely applied the ashes for making a cement instead of lime. Thus the rabbins trifle in their usual way, for when an obscure place occurs, immediately they invent some fable; though there be no history, yet they exercise their wit in fabulous glosses. What need is there of running to allegory, when we may simply take what the prophet says--that “the body of the king of Edom had been burnt”: for the prophet simply charges the Moabites with barbarous cruelty. To dig up the bodies of enemies, and to burn their hones,--this is an inhuman deed, and wholly barbarous. But it was more detestable in the Moabites, who had some connection with the people of Edom. If any humanity existed in them, they ought to have restrained their passions, so as not to treat so cruelly their brethren. When they exceeded all moderation in war, and raged against dead bodies, and burnt the bones of the dead, it was extremely barbarous conduct. The meaning of the sentence is this: The Moabites could no longer be borne with, for, in this one instance, they gave an example of savage cruelty. Their treatment of their brethren, the Idumaeans, proved that they had forgotten all humanity and justice. (John Calvin.)