Some clubs have colours which are national colours, and their fans may use a national flag to show their support. eg Watford have colours which are the colours of the German or Belgian flags!
As an Irish supporter of Arsenal i have to say the ban is ridiculous. It hurts the English Fans aswell as the Irish/Scots etc lucky enough to get hold of tickets and who want to show the club/team its support by bringing their national flag to the game
I'm beginning to wish we'd never moved to this stadium now. The stewards seem determined to make examples of Gooners who stand up and sing, while ignoring the away support. Ludicrously priced category A games, over priced food which is still of an extremely poor quality and now the flag ban. What next?