@Henrik ****
yes you are correct, especially in airside shops. I think with SIM cards however, most people assume that the True mobile and AIS shops, after you clear customs will have the same deals available as those in central Bangkok, when in fact as you rightly advise, they do not. I have always bought a tourist SIM card until I found out about this and bought a 1 year AIS deal for about 45 usd.