I don't know if it is gross authoritarian control. The over arching point is to check whether people report themselves as being on vacation when they are. Your employer would also object if you forget to tell that you are on vacation rather than working If they found 21 people in one day of checking, there might be a point to carrying out these checks.

The reason for selecting non schengen (and most likely non EU) flights is that people could have been seeking employment within the EU,and that would be a perfectly good reason for being out of the country, free movement and all that. So being checked on a UK arrival is not likely.