Annoyances – Part 2: Spitting

I never need to spit, ever. So I don’t understand why so many of the people here do. During our two hour lessons students will frequently ask to be allowed to go out so that they can spit. During the rosary in the evening there will be a steady trickle of students walking over to nearby bushes in order to spit. It is quite normal to see people spitting in the street. I don't get why?