Why did Metallica make a song called dont tread on me?

The words in the title of this song are from a historical American flag from the mid-1700's. The flag showed a rattlesnake with 13 rattles - symbolizing the 13 colonies and had the words 'Don't tread on me.' on it. The meaning was that to bother the new united colonies was as dangerous as stepping on a rattlesnake. If you listen to the words of the song, it talks about the same idea - that the US will defend its traditions and the freedom of it's people.