Some airports have "International" in their name, so maybe that's why it shows up. If you're ever in Tampa and read the paper or listen to the news, you'll see/hear them refer to "TIA". Huh? Don't you mean "TPA" (as anyone who actually flys knows)? No, it's "Tampa International Airport" so the media insist on referring to it as "TIA". That said, I just looked at some old boarding passes, which just say 'Tampa' - though they do say 'Miami International'.

I once did some work in Greenville, South Carolina; used to fly in/out of "Greenville/Spartanburg International Airport". Because of some industry in the area, I think there was one international flight a day (once a week?) to Europe. I guess it makes the people feel better if they have an 'international' airport.