Malbim: There were different Avodos. Some idolatries they loved, and attributed to them good influence, beauty and all success. Some they feared, and attributed to them all illness, plague and sword, and they served them via Korbanos and human sacrifice to assuage their anger and remove bad occurrences. Some they followed to mimic the Midos and ways that they attributed to them, and some they probed to know the future and hidden matters. Some they merely bowed to, to elevate them and honor them.

Malbim: It was normal to gather the bones of nobles to honored coffins. These will not be gathered; they will not be buried at all.

Rashi: It is fertilizer. Radak - they will remain unburied so long that they will rot and return to be earth and fertilizer.