The article is thin on facts.
The CA DHS monitoring the Minutemen can be constured a lot of ways. Let me throw on on the pile that may be close to the truth.
Organizations change, evolve. Many times factions split off, those factions being more extreme that the original organization. The Minutemen have already had some pretty extreme types show up at the door looking to "help" and they have done an excellent job keeping the extreme elementts out.
If CA DHSis monitoring the MM, it should to look for signs of factions breaking off or infiltration by the more extreme types. IMHO, That would be prudent. Monitoring them otherwise would not.
Please forgive the typos-
Coffee hasn't kicked in yet