Certain SIM's are only available if you are a Thai national or "hold a work permit" (standard line from DTAC/True/AIS customer services officer). So, the foreigner has a SIM in a trusted Thai friend's / partner's name for convenience.
You could have asked politely but instead you asked in a condescending interrogative manner as ever. Could you tone it down?
You're referring to post-paid accounts? I'd guess that most foreigners don't use those accounts, they buy pre-paid SIMs to which the OP refers. Your interpretation of my perfectly normal, non-condescending, non-interrogative question is wrong. Could you please tone down your rude assertions?