First, let's look back at the Bill of Rights:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances. (emphasis added)Now, from the Supreme Court ruling in Texas v. Johnson (1989):
Under the circumstances, Johnson's burning of the flag constituted expressive conduct, permitting him to invoke the First Amendment.Finally, in response to Trump's insistence that flag-burning could be punishable as treason, I quote once again from the Constitution, this time from Article III, Section 3:
Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort.So, the tl;dr - flag burning is not illegal. It's free speech, and no law can be passed to make it illegal unless the Supreme Court reverses that ruling. So any new law would be struck down as unconstitutional. In the unlikely event that the Supreme Court reverses its ruling, any law that calls flag-burning treason would also be struck down as unconstitutional.
And that, my friends, is how it works.