I think it all depends on the room host and stuff. With the "UK Only" rooms, I guess it's because the UK guys want to hang out with the other UK racers. Personally, I'll join any room that interests me, including American ones. I have friends from all over the world, from Austrailia to Germany to America, so I don't really care where you're from, as long as you don't piss me off, I'll race with you. lol
I've only restricted the 'type' of people in my room once... and that was "Noob drifters only", because I wanted to practice drifting without having the pressure of all the good drifters out there watching me, I thought it would be a room where all the 'noob' drifters like myself could just hang out and crash together lol.