But the compiler doesn't see the format string. The format string is entered as a parameter to an already compiled (exploitable) program (usually taking advantage of printf(string) instead of printf(%s, string)). So the compiler itself does not ever see the format string because it is entered as a parameter. I am almost 100% sure that this is to ensure that the data being entered (the address to be stored) lines up correctly in memory (dealing with word size).