Many of the street vendors come from the surrounding areas. They are essentially trying to sell anything they can to make some kind of money to support their families. A lot of time you will see the children with the mothers, which makes me heartbroken to see such innocent children having to sell items just to survive. If you see people, such as these in the picture, please be kind and help them out. What little money you give them could help to pay for a meal that night.
Local Tip: The Ejercito Zapatista Liberacion Nacional (EZLN) sprung up as a guerilla movement in the 1990's. The movement was created in Chiapas to help improve the lives of the local indigenous population. Sadly, many of this people leave well below the poverty line. The guerilla movement has cooled down in recent years and I really doubt it will hinder your travels. Nevertheless, check out the current situation before heading into Chiapas. For additional information on the movement, check out their webpage at http://www.ezln.org.