Lots of people from across the region support NUFC it's by far the region's biggest and most supported club. You have to go as far South as Middlesbrough to find another club in the top two tiers of English football. Further north than Middlesbrough, NUFC represent the regions only top club. There are a number of lower league sides such as Gateshead, Hartlepool and Sunderland but it's reasonable to say they aren't at a higher enough level to draw in the levels of support that NUFC do.
Also people from Northumberland, Gateshead and Durham will have contributed towards the flag in very large numbers so it is only fair they are represented alongside those from Newcastle and Tyneside.
Links to other clubs' main forums
Re: Links to other clubs' main forums
Tyneside includes Newcastle, Gateshead, Tynemouth, Wallsend, South Shields, and Jarrow. I really don't think its a big deal the the club in Newcastle upon Tyne is associated with places in Tyneside. No worse than Manchester United being associated with the City of Manchester, as a club based in Trafford.