To burn the {flag} and replace it with a Parental Advisory sticker

from Eminem – White America Lyrics on Rap Genius

Meaning

Flag burning was a hot and divisive political topic around the time this song was released. Flag burning is protected under the 1st Amendment as free speech, which the government can’t (not for lack of trying) regulate based on content.

Em is saying, you are (illegally) censoring our powerful right to free speech by replacing the symbol of the American flag with the content advisory label:

Ironically, this line itself was censored, with the word “flag” left out

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