I don't have all the facts, but I heard agreed with you say here, They were arrested in disputed zone for immigration violation by Cambodia.

The fact that Thai government fails to protest strongly is viewed as acceptance of Cambodia's sovereignty over the disputed zone. This is a grave foreign policy errors amounting to a loss of Thailand's sovereignty over the same zone. Thus, admission of Cambodia's sovereignty --by default--.

Thailand must protest by diplomatic means, by trades, by sending back and stopping illegal Cambodian migrants, by closing border with Cambodia, by banning all exchange programs,...

Cambodia must learn to respect Thailand's sovereignty again.