All 50 states have laws against militias and paramilitary groups, but most don't enforce them and there are large militias in many parts of the US. Why not? It seems like a fairly important state interest to maintain public order against armed paramilitaries.
The laws don't seem to forbid private militias and military units outright. According to an article in the Wall Street Journal:
But all 50 states “prohibit private, unauthorized militias and military units from engaging in activities reserved for the state militia, including law enforcement activities,” according to a fact sheet published by the Institute for Constitutional Advocacy and Protection at Georgetown University Law Center.
Some states, including Michigan, also ban “paramilitary activity during or in furtherance of a civil disorder,” according to the Georgetown center.
“In the United States, it is not illegal to be a member of a group or movement which has extreme views,” said Tom O’Connor, a retired Federal Bureau of Investigation special agent who investigated domestic terrorism cases. “It is only when those views are taken to the level of serious criminal violence that the act itself becomes illegal.”
Often, the reason that these paramilitary groups are not prosecuted is because they don't violate the laws in the states where they operate.
If they do break the law, then there are a number of reasons why they don't get prosecuted:
Prosecutors may not be aware of some specific laws because they are rarely used.
These cases may be hard to prove because laws that about disturbing public order may require them to prove specific intent.
Again, quoting from that Wall Street Journal article:
Prosecutors are often unaware the laws are on the books, legal experts said. Such laws “are not the bread-and-butter criminal laws that they are used to enforcing,” said Mary McCord, legal director of the Georgetown center.
In addition, “the required legal thresholds are often difficult to meet because they generally require proof that the suspect activities were motivated by an additional specific purpose to foment violence or civil disorder,” said Brian Levin, a former New York City police officer. Mr. Levin now heads the Center for the Study of Hate and Extremism at the California State University, San Bernardino.
Instead, in specific cases it may be easier to bring other charges. As the Wall Street Journal article continues:
“Sedition cases are very difficult to prove,” said Peter Henning, a former federal prosecutor and now a law professor at Wayne State University Law School. “Charging the defendants with kidnapping is a much easier road to go down.”
Put simply, we have the same problem outlawing private militias that we have outlawing street gangs. The right to freely assemble is enshrined in the first amendment of the US constitution, and so long as people are not engaged in explicitly criminal activities that fundamental right is extremely difficult to abridge. They might call themselves militias, but in the eyes of the law they are merely civic associations (no different in principle than the Benevolent and Protective Order of Elks, or the Rotary Club). One of the fundamental principles of US law is that the law cannot prosecute someone unless a crime has been committed, and (if I may speak frankly) a bunch of guys sitting around playing with their guns and talking about insurrection are not committing any actual crime.
When these militia groups try to act like actual militias — trying to prevent crime; standing up to civil authority; declaring their sovereign right to self-rule; etc. — then they begin doing things that are interpretable as crimes in most if not all US states. Such groups frequently face state or federal interdictions (everything from Waco and Ruby Ridge to the Malheur Wildlife Refuge incident). But until they step over that line into action they are usually tolerated, if not outright ignored, because they have the cloak of the First Amendment's right to assemble peaceably.