Why does justice brennan agree with gregory lee johnsons feelings about the flag?

I hope this answers your question..

The majority of the Court, according to Justice William Brennan, agreed with Johnson and held that flag burning constitutes a form of "symbolic speech" that is protected by the First Amendment. The majority noted that freedom of speech protects actions that society may find very offensive, but society's outrage alone is not justification for suppressing free speech.

In particular, the majority noted that the Texas law discriminated upon viewpoint, i.e., although the law punished actions, such as flag burning, that might arouse anger in others, it specifically exempted from prosecution actions that were respectful of venerated objects, e.g., burning and burying a worn-out flag. The majority said that the government could not discriminate in this manner based solely upon viewpoint.

So far i know that you cant