The Zetas are on the outs with the Mexican government which has been helping the Sinaloa cartel. So the Zetas have sided with the Juarez cartel against the Sinaloa cartel and the Mexican government. The Zetas were responsible for attacking simultaneously two Mexican military bases in retaliation for aiding the Sinaloa cartel kill over 25 Juarez cartel gang members.
The free farm work demanded might be growing marijuana. Illegal immigrants are routinely exploited. That is why Phoenix is the kidnap capitol of the USA.
EDIT: In a perfect world where the U.S. Govt was what it was intended to be we could stop the border murders.
The United States Govt should give notice that unless the Mexican Govt does something immedietly to halt the murders along the border the United States will declare war with the intent to annex.