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.