99.9% of politicians are scum, but that's not the point.
The point is that if politicians are able to just be killed willy nilly with no protection, particularly the president, you could quickly end up with a Banana Republic situation or a major dictatorship because eventually the screws will tighten on that sort of thing. The other possibility is that you end up with people doing political things out of fear (as opposed to our current system of simple bribe..err.. lobbying).
The USSS and organizations like it, including your basic cops acting as bodyguards for a mayor or whatever, do so because at the end of the day it's not about the person they are protecting, but the office they are protecting.
I know a retired Secret Service agent. She was tasked with VP detail during Clinton/Gore and Bush/Cheney. She despises the Clintons, the Gores and that whole lot of folks from that administration. She preferred the Bush admin as people, considered as such. But her job was to protect the VP and his family -- not because they are worth it as people (they're scum), but because of what their assassination would have represented to the system of a Constitutional Republic (dead) we (supposedly) have.
In other words, it's not a micro reason, subjectively focused; it's a macro reason, objectively focused.
What the USSS agent in the article did was lose her objective focus and make it about the person, and that's decidedly bad. She needs to get fired.