Fraudulent voters? Nah

Rico says his friend Robert sends this from California:

I talked this morning to one of the officers from the Butte County Inter-Agency Task Force, who was tasked with evidence collection at a recent pot bust up the road.  In addition to, literally, tons of bud, two Mexican nationals (excuse me, two undocumented immigrants) were arrested.  This is a major problem, as these cartel growers have no interest in responsible land management, nor do they care what damage they leave behind after harvest.  But what prompts me to write were two letters found at the scene from the Butte County Registrar of Voters, each containing a voter registration card. Both men apparently voted Democratic last June.
Of course, local Democrats say voter fraud isn't a problem.  Okay, but I have a question the Romans used to ask: Cui bono?