If espionage can hurt other players, then it starts to feel annoying that a minor technologically backwards civ can be just as effective as a superpower (the only way they are weaker is if they are a full era or more behind). The US just doesn't need to be worried about espionage from Bhutan.
technically Bhutan is 2-3 eras behind US.
Further, US is bothered about Ukrainiang or Czech or N-Korea hackers, which are kinda like spies, from low-tech, low power civs/countries (they don't have the capability to build high-end techs, or even computers... they buy it, mostly)/
if in reality, US is not-so bothered, it's because the Espionnage/counterespionnage force of US is huge ; not because US army or US economy or US tech lead is Huge.
so yes, powerful countries that don't want to be bothered by small countries spies spend a lot of money to have more spies or more efficient spies than the others.
otherwise I agree with you that the effects of water poisoning or else don't happen in modern era ... but IMO, it is because counter-espionnage is well developped.
last, I think that espionnage could also represent terrorisme as in :
-killing a leader (palestine, bin-laden, those plutonium-killed guys, JFK...Etc)
-extrading/kidnapping/killing a scientist/artist (happened all the time in cold war
-bombing /gazing cities, (IRA actions, subways gazing, planes on 9/11...Etc).
true, those actions are not "reported" to be linked to a foreign COUNTRY.
however, those happened, and they could (mechanicaly speaking) have been controled by another country.
further, Cold-war induced a lot of those kind of actions... mainly in non-aligned countries (because easier targets), but both URSS and US (and UK, FR...Etc) did it.
they even did it a lot pre-WW2, in the colonial era.