Wouldn't it be a volunteer post in the first place?
If so, wouldn't said volunteer have to apply for the next open slot? If you don't want to be a human shield for a human shitbag, don't submit the application, right?
I wasn't able to find anything concrete about whether or not the POTUS detail is volunteer. In all honesty, and knowing the gummint, I would think that corporate/Secret Service politics would enter into a situation like that at least to SOME level.
Seems to me the gig would be offered based on lots of different criteria, not the least of which would be what you'd expect an SS agent to be -- watchful, competent, ego-less, diligent. But there is NO organization yet devised that doesn't have its own "methodologies" in appointing people to particular jobs.
Sure, I'd think that anybody who was to be appointed to the POTUS detail would have a say-so in the matter. But the attempt would be made to recruit/offer the gig in the first place. And there's no doubt in my mind that there are those who, because of the prestige associated with the gig, would defend Pol Pot from Cambodian survivors of the killing fields.
OTOH, how embarrassing would that be for the SS supervisor to offer the POTUS detail to an agent who calmly tells the supervisor to go **** himself? That he wouldn't defend the next POTUS from an self-inflicted attack of the farts?