Well, for one thing the cartels have forced parts of the civilian population -- especially in Michoacan-- to work for them (farming poppy; processing raw opium; spies and lookouts). These people are then affected by narco inter-gang violence, since they are viewed as part of the organisation thay are forced to work for. This also explains the large number of "gang-members" killed, when the mexican state publishes numbers on the death toll of the drug war.

Another thing is a system of "taxation", narco gangs put on these people. They have to pay the taxman from the state, as well as protection money to the rackets.

For a great documentary on this topic (this and how the people are trying to defend themselvs) I can point you to "Cartel Land" -- available on Netflix.