Let me give you an example. I claim:
It's always dark in USA, but only at night.
What you did, you essentially jumped onto the first part "It's always dark".. Ripping it apart... With your current logic you'd call me outright stupid, because I said, It's always dark. The modifier that completes the sentence, gets totally ignored.
Look at GC's sentence again. Take a deep breath, and then read it again with proper grammar application. GC followed grammar rules and put a comma before the "but", yet that entire part belongs to the sentence and changes it's meaning entirely.