Language soup. Chai stall or soup stall?

Though a Tamilian, the way the purist write non-Tamil words in Tamizh  has always amused me. Does the Sai stall sell Chai (tea) or Soup? A scene from Luz, Mylapore. 
Note the Valai thantu soup. Isn't the sound 'zha' the pride of Tamizh? Then why make 'vazhai' 'valai'?