Just a guess. I think the use of "fossil" ivory on modern instruments is due more to tradition (and appearance) than to any properties of the material. Ivory used to be put on piano keys but plastics are used now. Billiard balls used to be ivory (I remember being permitted to use ivory cue balls at a local club many years ago) instead of the new, much cheaper composition balls); what differences they had in response to those of modern balls I don't know.

The Canadian Inuit use large quantities of "fossil" ivory in their carvings. Some is obtained from Russian sources as well as Canadian. Canadian seal fur products are barred from entry into the U. S. but whether these regulations extend to "fossil" ivory, I don't know. The carvings may be sold in Canada and into the Japanese and European markets. Canada follows the U. S. position on eagle feathers (can't sell any artifact bearing eagle feathers, regardless of age or provenance- some cases in the appeals courts now).