I dont know if this is entirely true. I have sold many eggs and birds out of state and haven't had to be NPIP certified. No one ever stopped me. To show birds, yes you have to be certified.I wish I had only paid $3 a bird. The color/breed I have sell for $8 to 10 per chick or 20 for adults. There are only a small handful of breeders willing to ship. I found only 2 or 3 in the US that will sell shipped hatching eggs, 3 sometimes 4 that sell chicks shipped. When you have a few hundred dollars wrapped up in chickens the vet bill for testing is worth it.
You are right if I had practiced better bio-security I may not be having this problem. My adult coop seems very healthy, active, and looks great. So I could assume that I didn't pass MG from my baby chicks to my adult coop. But would you buy chicks from me knowing that my baby chicks were sneezing and all I did was medicate them? Or maybe I should say nothing and do nothing because its common and they don't look or act sick. I'm not trying to get nasty but there are only two possibilities I got this, either from an infected person coming on to my farm or the adult chickens I purchased a few years back as my starter flock already had it. Either way the chicken owner had to know they had sneezing chickens and didn't care enough about other flock owners to find out why their flock is sneezing. Or maybe they were new to chickens and thought it was normal. Either way if we can pass along our information we can help reduce the spread.
I only told my story because somewhere somehow my chicks got this disease. If more people new to chickens knew about the disease and how to prevent the spread of it, we wouldn't be talking about it. The problem is that you hear "when you do bring new birds in don't introduce them into your flock until after a minimum of a two weeks" which is only half of the new flock introduction. The other half of that statement is: take one of your flock members and put it with the quarantined new flock for another two weeks. The new chicks/chickens may not show any symptoms of the disease until after you put them in your healthy flock.
One thing I do know is that more people are likely to buy from a flock if they know they are not getting any diseases. I also know that in order to show a bird (or sell chicks, eggs, or birds outside of Michigan) it must be NPIP certified. Some states even require MG testing before being able to ship. These laws are not to cost us extra money or inconvenience us, they were created to protect chickens from spreading deadly diseases and wiping out entire breeds/flocks.