The Minutemen have never tried to replace law enforcement.
They have tried to aid law enforcement by providing them with information.
I haven't seen any evidence that the Minutemen have interferred with the border patrol or with law inforcement.
I'm happy to see that law enforcement is getting additional funding, however no rational person can argue that the border is even reasonably close to being secure.
The Sheriff may not want the help of the Minutemen. However, they aren't breaking any laws. They are providing information to law enforcement and in doing so they are also providing information about what law enforcement is doing and what they are failing to do.
A Sheriff who is trying to do his job should appreciate that more exposure on this topic should help him get the resources needed to attack the problem.
One should question why he doesn't want citizen envolvement. Perhaps the Sheriff has something to hide.