There's a lot of pissing and moaning from the "border security first" crowd on this thread, but we shouldn't forget they're complaining about the predictable result of what they themselves asked for. If you militarize the border, and federalize the military on the border, then the Posse Comitatus Act prohibits those military forces (excepting the Coast Guard) from engaging in any sort of policing activities. They can't arrest illegals. They have to call in the border patrol. The Guard in this case appears to have done the correct thing. And it appears that the Guard IS helping border security with improved observation and beefed up logistics. But you're still only going to hear the bitches from a certain FR cohort.
then the Posse Comitatus Act prohibits those military forces (excepting the Coast Guard) from engaging in any sort of policing activities. They can't arrest illegals. It depends on their orders. If the National Guard are wearing their militia hat, then they can, as they often are, be used for law enforcement duties, and are not subject to the Posse Commitatus Act. That is in the Constitution, Art. I, section 8, which the PC act does not override.
In any event, PC is just a law, dating from the 1870s. If we need to change it, Congress can do that by a simple majority vote.