They aren't allowed to here. Learn the laws of this country.
The federal law needs to be changed in order to use the military as a border guard.
No it doesn't, that's exactly what you totalitarian pigs would like. I'll endure 100,000,000 more illegal aliens before Posse Comitatus is repealed.
The Ninth and Tenth Amendments retain the the rights of the People (local counties) and States to secure that border.
One Constitutionally valid way to put troops on the Border would be for the Feds to "erect forts" every 50 miles that would be "Necessary and Proper" to uphold it's Article IV obligation to secure the United States and assist local and state authorities when the local/state executives request their assistance.
All of that can be done WITHOUT the repeal of posse comitatus. Get lost commie boy.