This line of "thinking" makes no sense...and kinda explains, in part, how the US got here.
Employees don't get to pick and choose what work they'll perform.
The secret service doesn't get to pick and choose which POTUS they'll protect.
Just like everyone else, do your job, or quit. No one gets to refuse to do their job, and keep it.
Let them defund themselves and lose the contract, call in the national guard to work security.
I think it makes perfect sense. There are two aspects of this decision. The first is an emotional one, and you have the right to not like it just as others have a right to like it if they choose - its completely subjective. That part is the "You want to defund me? Then I won't protect you." Totally understand your point on that, even if I quietly laugh at the irony of the decision.
The other part is the more logical and hard to argue against point. If you remove the tools someone needs to perform a job, you can't rightly expect them to do their job - especially if it endangers them in doing so.
If we tell firefighters they aren't allowed to use water, but still have to put out fires, they're going to look at us like we are complete morons and refuse to do their job.