A checkpoint is a type of military installation. Specifically an installation with the purpose of monitoring and controlling traffic traveling through the checkpoint.
A check point is a point where people or things are checked. It could be military or civilian. It could be permanent or temporary. It could be 1 person and a vehicle parked in a road. This would not be a military installation to most people. Or dictionaries. Or laws.
And the point remains, regardless of semantic quibbling: a checkpoint is probably the least surprising place to encounter surveillance. Arguably, it's the whole point of a checkpoint to surveil, "checking" in your terminology, who's passing through.
