The US doesn't expel allied diplomats, because they don't care if the French and Spanish and Germans are spying on us, _in general_. Most of the time countries get annoyed at allied spying, they'll make some quiet calls to their ally and have people quietly reassigned, rather than make a big deal about it. As long as the country knows who the allied spies are, that's good enough. What happened here is that the US spies were very clumsy and got caught committing crimes.
You're asking to prove a state secret. That the spanish were caught expelling allied spies is almost as much a faux pas as it is for your spies to get caught.
Also so t forget literally the entire Snowden era. Literally the biggest event geopolitical event between allies in the 2010s. I swear you guys choose to forget these things
The US tried to infiltrate Spanish state security. That's a direct threat to Spanish sovereignty. Imagine if they had succeeded and it led to a manipulation of Spanish policy, against the interests of the Spanish people.
What can Spain realistically do if the USA actually becomes hostile to it? The whole EU has already relinquished its full sovereignty a long time ago. The USA does as it wants and at most, the EU can expell some spies caught in the act as quietly as possible to not anger Big Uncle Sam.
There are many countries to which the US is actually hostile, and which are much weaker than Spain. Venezuela, Cuba, etc. Places like Russia and China also of course exist.
Spain could start trying to compete with the US in South America, they could push some kind of EU South America expansion, some kind of EU analogue of BRICS-for-South-America-but-not-BRICS; and of course, if it truly became a problem they could decide to go much further than that and start being actually hostile themselves, joining with the whole RIC part of the thing.
Why would the US bother? Unless they propose not to engage in diplomacy, the replacement diplomat is going to spy on them too. It isn't like these are rogue actors, the spying is sponsored by their state.
The only reason to even talk about it would be if they want to roll a particular diplomat who is hard to work with or if it was convenient to gin up a diplomatic incident so that they could pretend to be unhappy.
The US does expel diplomats who engage in spying. For example, in 2022 they expelled twelve for spying in ways that threaten US national security.
Even if you try to keep an eye they will of course try make keeping an eye on them difficult. The US feels it can spy-- with people having been found with wigs and whatnot, so to imagine that others can't, even if they are allowed to do things very freely is a little bit strange.
When you say "in 2022" do you mean the ones listed on this page [0]? The "Diplomatic expulsions during the Russo-Ukrainian War" page? I'm sensing there might have been issues at play other than the espionage, which is routine and everyone does.
Notice though that the US specifically chose to give a reason for the explusions as espionage activities. Spain expelled those it expelled over 'Russia's terrible actions'.
I think we can take the reasons for the expulsions as correct.
The reasons are honest insofar as yes, the Russians expelled were involved in espionage. That part that has me tagging you as naive is you seem to believe that this was news in 2022.
It is the Russian embassy. The US spooks are monitoring it very closely. It is not a serious position to say that they suddenly discovered that espionage was afoot a few days after the invasion. I mean, it is completely plausible that the invasion made them say "we're about to hit the Russians hard, now is the time to disrupt their espionage activities". But the existence of those espionage activities has been a given since for the entire history of Russian diplomatic missions in the US.
The US is certainly not going to expel diplomats for spying under ordinary circumstances. We can see that because places like the Chinese, French and British and whatever other embassies are still allowed to exist. Maybe there are some African or Pacific Island embassies that are incompetent and just exist to chat, I dunno. But we know how the serious players use embassies this has been a thing for centuries.
So either these countries don't use diplomatic cover or they don't spy on the US.