It is a ridiculous argument, and I'm not making it at all. I'm just pointing out that the reasoning is similar, yet one is fine, and the other is ridiculous, as you say.
I agree, the appeal is what makes it have value. There's no legislation that can remove that appeal, so there's no real way to ever completely devalue it. Value = demand = poaching and we go round and round.
I have no objection to the importation ban of ivory. If you already own it, and it is here, it should be a free market. I also do not believe that mammoth should fall under these same sanctions.
To address the loophole that it could create, you could easily make a register for preowned and existing ivory that's in the USA already. It's not as massive as a task as you would think. I could create a website tomorrow that would allow the end user to take a couple of pictures and write a description and submit it online, and receive an ID number. - Advertise this method, leave it open for a year or so, and than close it down.
The idea is that if it isn't able to be traded, it can't be monetarily valued or acquired, so the appeal dies with the owner. And with no trade, there is less discussion, books, videos, etc. When a country whose culture is so closely scrutinized by other nations as the US stops taking an interest in something, that has an effect. Doesn't matter if it is ivory or disco.