Inside a Mumbai local train coach

I often wonder why it's so easy for people to post their advertisement posters/stickers free of cost in the coaches of local trains in Mumbai. The coaches of the local trains are full of such advertisements, most of them being of sex clinics, spoken English coaching, computer classes and medicines. Some of them often happen to be the advertisement of some baba's or Devi's discourse. They spoil the inside appearance of the train.

No one - neither the citizens nor the railway officials - seems to be bothered.