I have not YET seen any BYCer getting into trouble of NOT having NPIP across the state lines. Come on, the officials are not everywhere and they can not monitor every bird coming in or out of the state. I can see if it is for show birds and have them tested for diseases common in the US and for the peace of mind.
I was on the fence and ready to get my birds NPIP until I did some research with both NPIP and private breeders and the USPS. It is not cost effective for me and believe it or not, many of the "importers" back in those days didnt have NPIP with importing birds or eggs into the US illegially and some STILL do it this day. With thousands of folks coming and going, it would be like finding needle in a haystack however COMMON SENSE says do NOT ever try to do it. If you get caught, you can have your birds put to death, fined and jailed. As for eggs, they would confinsate it and destroy it without the legal paperwork.
With US interstate between states, it is not justified in stopping all animals to see if they got paperwork but events and shows DO require Coggins or NPIP or vet's certificate for all livestock is common sense and protect you and the exhibitors if any of your animals or theirs show signs of illness. It would require detective work to nail the person who spread it but its too hard to nail everyone down, even the public unknowingly brought it in by hands, clothes or shoes.
All the birds I've sold and most of them were 4H and one exhibitor say all of my birds came out clean as a whistle, no diseases and tested negative across the board in five different common poultry diseases. I thought the tests were a bit of an overkill but they insisted on it. They have done so well and I can not be even more pleased about how they are winning the awards for young people and exhibitors alike.