TABC works with local law enforcement and arrest you red handed at the moment you sell or serve alcohol to a minor. However, it's not true if you are tabc certified. That means if you have taken the safe alcohol serving class either physical or online and passed the test, got certificate and you present to the officer at the scene then they do not handcuff and take you to jail, rather write a citation and summon you to the court. But as the original poster said, he/she was not arrested at the scene, then I suppose he/she was already tabc certified, or worked in an exempt county, or officers made an error, or may be just got lucky.
To all who has to sell or serve alcohol, please get certified. If you don't have time, you can also take online class. Go to learn2serve.com and take the class. It only takes about 3-4 hours. And its not boring. Its fun with all the animations and conversations. The best part is you will sell or serve alcohol more confidently the next day and after and you will be at less risk. Be ABC certified and be a responsible server today. For those in Texas, use txonline as promo code at learn2serve.com to get $5 discount out of $25 dollar (not quite sure but less than 40) class fees.