The point is, the trophy trade legitimizes "trophies" as something of value, which is something countries like Kenya and Botswana are actively fighting. They burned 100 tons of ivory in 2016 alone - for the same reason the DEA destroys heroin. If they sell it, they legitimize the trade. They don't think it should be something of value, therefore they destroy it. I'm sure the Trump bill includes some things I don't give a shit about like zebra skins, but specifically regarding elephant and rhino parts, it undermines the effort of countries like Kenya and and Botswana and plays well with kleptocracies like Zimbabwe that allow trophy hunting.
Of course I don't believe for a second that this is something that Trump thinks of in conservationist terms. This is a gift for his son because that's how politics work now.