I think Penetrator has hit it on the head. You may also notice other inconsistencies in rural Thailand. Eg Kids are allowed to ride bikes to and from school (with or without a helmet) but they are likely to get pulled over by the cops if they're riding after school hours or clearly not going to school. Same goes for registered bikes. You can usually get away with riding an unplated bike on the village streets in Bangkok or pretty much in any province but if you ride it out on the main roads look out!
Just my observations after 17 years of hanging around this part of the world. So probably not worth much.
i,m not gona wear 1 if i can get away wth it(main rds ok)like the feelin of wind in my hairs!-now!i,m 44-not so young julian!,shld know better tuttut,after years of wearin in uk i,ll take chance til novlty wears off.