The number of elderly people in my local Thai hospital is massive and many are there for simple ailments that a few baht in a chemist would solve. They actually get upset if a doctor tells them that they have the cold & should stay at home, have hot drinks and stay warm.
If the doctor doesn't prescribe them something, they feel slighted and let him/her know. Many docs just prescribe them vitamin A to get them on their way.
Maybe the small payment of the 30 baht might make them think before they make an unnecessary trip.
If something is totally free, people will abuse it.