"Ultimate source" in parliamentary democracy just means that people vote (and once in several years).

They don't influence the government in its day to day operations, nor they have any special power or significance for the various policies as individuals (outside of their function as a voting majority).

So while this might make them a "legitimate" target for terrorism (in the sense: they voted for the government so they are ultimately responsible for its foreign policy etc) it doesn't make any sense at all regarding to espionage.