Why was my son questioned at immigration after completing an education visa in Thailand?

My son completed 11 months of a 12 month education visa for Thai language. I was on a retirement visa for a year. My visa expired a month before he was due to finish ( I got it through the Canberra embassy and it arrived faster than I thought). So he got a letter of early finalisation apparently required from the language school in CM. We then departed for another country on the day my visa finished and were gone a month. We checked everything face to face with CM immigration about leaving and then coming back on a tourist visa, returning for 3 weeks before heading home to Australia. We arrived back at Suvarnabhumi airport and no problem I was stamped through. Then they stopped my son and I came back with him and he was given a hard time about not finishing his course. He had the letter. He was asked one question in Thai and the officer was saying he hadn't studied. We had just come off a night flight without sleep. Then the officer said he cannot come back on an education visa. There were three officers. They were pretty rude and officious but then did allow him to be stamped in as a tourist. We did all the right things according to CM immigration requirements but Bangkok were upset. They also said we cannot come here to live etc. Never said we wanted to!! I love Thai people in general but is this usual?