US citizens have freedom of assembly. If there's a concern that the militias are armed, are espousing hate or other crazy ideology, they should be monitored, but if they aren't breaking laws or hurting anyone, they have the right to form. They don't have the right to refuse to pay taxes, to spout hatred at others, or to harm people, but they can believe whatever crap they want to. It's their right as Americans.
I think civil militias are a consequence of the gun laws and they will always be something to watch for in the future. Any crazy but charming leader can amass enough followers to be a danger to the country or a state or even city. It is good citizenry to watch for the developing of those.
Bearing arms and forming militias is a fundamental right in the United States, founded in the Constitution. However, this right was really only meant to apply to people in times of war and violence. If your individual safety is a stake, you should be able to bear arms and form militias in order to fight. If you feel that your government is threatening your safety, you should be able to bear arms and fight that government. However, in these particular circumstances, no ones safety is at stake. Therefore, militias should not be allowed to be formed.
We should not allow militias to form in the United States because militias undermine law enforcement by taking justice into their own hands. The United States has a good structure of local and federal law enforcement and the strongest military in the world, so militias are not necessary, and often they go against the law because they disagree with it.