That would be because he violated the terms of his probation. He was charged with exactly that - a probation violation - not with a new crime of possession.
You make a habit of being dead wrong, don't you. The crime charged in DUI is driving while impaired due to the consumption of alcohol or drugs, not possession. DUI laws very often are alcohol-specific. Some states separately criminalize operating under the influence of drugs, often with much stiffer penalties. But again, the crime is not possession - it's driving while impaired. This is far from complicated.Quoting Scooterdog

