In both recent cases, the individuals were arrested in the Phuket airport departure hall (for one of them, it is specified that it was the international one) by the immigration police. My first thought is that they must prefer to arrest them there than at a previous stage, possibly to avoid delays or cause a stir, or maybe to give them time to alert and deploy the appropriate team (I doubt the guy stamping passports in his booth can just step out and arrest a guy). And once you're in the departure hall, it's not like you're going anywhere so it's probably more convenient to make arrests there.
Hopefully there will be more details about this, but I don't see why it would just be a Phuket thing.