Comment by viraptor

> if there is a background level of foreign agent activity they accept

Yup, there's constantly a number of known spies in pretty much all countries. If all were ejected, the other country would do the same to their spies and monitoring compliance with projects would become harder - so it seems in everyone's interest to not be too strict. See for example https://johnsontr.github.io/assets/files/spies_current.pdf

Also there's the idea of "the optimal amount of fraud is non-zero" which generalises to lots of things - including this one.