If I'm understanding correctly, you've misread the statute. It says, specifically, that the use of the military to enforce the laws is illegal "except in cases and under circumstances expressly authorized by the Constitution or Act of Congress." The key word there is "except".

For instance, if the SCOTUS decides, through a process laid out by the Constitution, that school segregation is unConstitutional, the President can, legally, federalize a state's National Guard units to integrate schools, as Ike and Kennedy did after Brown v. Board. 

Would you support a similar action to round up and imprison illegal migrants?  Unless either your citation or my reading is incorrect, the Posse Comitatus Act of 1878 offers no protection against federal military intervention to enforce the laws in the states.

PS: It would be pretty hard to mobilize the Air Force in 1878, wouldn't it?