Haha, poor Hex Machine. Maybe Lamb of God hit the stage late because Richmond dissed them? Do headliners even care about the opening bands? I know the Chili Peppers defended Mars Volta after Omaha booed them off stage. Sometimes headliners say to applaud the opening bands and sometimes the opening bands aren't even acknowledged by the headliner.
The worst I've seen an opening band treated was in Buffalo when Anchored opened for Black Label Society (they were the first opening band of 3) and even then it wasn't that bad. The singer said something like "This next song is called 'You'" and almost immediately someone in the crowd yelled out "Suck!". Then towards the end of their set someone chanted "Free Bird" and the band took it all in good fun and said "No, we're not going to play that one"