The AP has a story explaining why militias pose a problem at the border. However, as you dig into the story, the reason apparently is that be using military fatigues and military gear, law enforcement can't distinguish militia from other law enforcement! From the story:
On a recent moonlit night, Border Patrol agents began rounding up eight immigrants hiding in and around a canal near the Rio Grande. A state trooper soon arrived to help. Then out of the darkness emerged seven more armed men in fatigues.
Agents assumed the camouflaged crew that joined in pulling the immigrants from the canal's milky green waters was a tactical unit from the Texas Department of Public Safety. Only later did they learn that the men belonged to the Texas Militia, a group that dresses like a SWAT team and carries weapons .... (Emphasis added).
... "How do they identify themselves? Do they have badges? How do we know who they are?" asked J.P. Rodriguez, a spokesman for the Hidalgo County Sheriff's Office. "If they're all just dressed in camos, it's kind of hard to distinguish whether they're law enforcement or not. ... There's a lot of potential for stuff to go wrong." (Emphasis added).
... Maricopa County Sheriff Joe Arpaio warned of "chaos if you're going to have private citizens dressed just like our deputies taking the law into their own hands." (Emphasis added).Maybe the problem is not the militia. Maybe it is law enforcement dressing up like soldiers. If the law enforcement wore actual police uniforms, they wouldn't have any problems figuring out whether militia were fellow officers or deputies.