The USEF did what anyone would expect: It found a way to not have to have an opinion. That allows two things to stand:
1) Not Mandarino's status. I don't think the Powers That Be-- the BNTs who make their living from ueber-quiet hunters care whether or not this one pony trainer gets to stay at the party. But they do need to know that no one will be coming from them next, should they need to use their quieting drugs. It's a "keeping up with the Joneses" thing in the arms race to get horses that are consistently so quiet. No one can afford to say no.
2) The USEF gets to keep its hard-to-enforce D&M laws to stand. Why do that? Because it's much cheaper to have the use of performance enhancing drugs be illegal on paper than to test and enforce that. Do you think membership will want to pay the costs of fencing in barns as happens at FEI shows? Paying DVMs to administer each and every injection given on the show grounds? Test, say, the top 3 horses in a class and random competitors?
I do think the USEF could solve their problem with some hard-a$$ penalties for being caught. And the additional rule that a horse who dies on show grounds is automatically subjected to a necropsy which the USEF controls and the person who signs as "trainer" pays for, if found guilty.