Why was my daughter sent to jail in North Richland Hills for Minor In Possession?

According to my daughter: My daughter and two friends were driving to a party last night and when they arrived it was too crowded so they turned around and were leaving. They were pulled over because she failed to signal the required 100 ft. before the intersection. My daughter was driving and is 18 and the other girls were 18 and 17. The police found a beer in the 17 year old girl's purse and took my daughter and the other 18 year old to jail and I don't know what happened to the 17 year old. It sounds really excessive to me to arrest them on something they didn't know about or I'm not getting the whole story. Who should I talk too to get the whole story and what should be done? My questions are: Why would they look in the girls purse, unless they suspected something? Was it because they thought they they were coming from the party?