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.
[0] https://en.wikipedia.org/wiki/Diplomatic_expulsions_during_t...
I think we can take the reasons for the expulsions as correct.
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.
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.