Hey John Free!
Watchtower is breaking its own rules on this matter in order to save money. In their own literature flags are referred to as 'objects of veneration'. Objects of veneration are idols. Also in their own literature it isn't simply the veneration of idols that is condemned but the very possession of an idol that is condemned.
When driving your car in another country you normally need to display some kind of national identification mark on the rear.
Also see:
https://en.wikipedia.org/wiki/Vehicle_registration_plates_of_the_European_Union
Also don't ships, boats and yachts etc need to display an ensign at certain times?
https://en.wikipedia.org/wiki/Ensign
Watchtower 15 January 1977, page 63, paragraph 4
Questions From Readers: May a Christian raise or lower a flag at his place of work?
Often the national flag is displayed at public buildings and assembly places, such as firehouses, municipal offices and schools. In view of their respect for the government and what is used to represent it, Jehovah’s Witnesses have no objection to being in or working at buildings where the national flag is on display. Similarly, a flag might appear on postage stamps, automobile plates or other government-produced items. Yet that does not mean that a person of the public who uses such is necessarily participating in devotional acts toward the flag. What is significant is not the presence of a flag but how a person acts toward it, what he does.